It's entirely possible that you need to replace an ethernet cable between your RPi and the Bell device. Just try inserting a different cable there, and see how that goes.
Those last results were from my laptop connected directly to a LAN port on the Bell modem/router. I even tried hooking up my pc on a separate port with another cable. I even hooked up the fibre directly to the Bell modem/router and disconnected the media converter and managed switch. I wanted to make sure nothing I added to my network could have affected my connection. Iām still convinced thereās āsomethingā else on Bellās side affecting my throughput. 
Hmm... is this Bell device needed? If you have a media converter that converts from fiber to ethernet, just plug your RPi directly to that?
(Note: on ATT's network, they have secret crypto certs in their device that authenticate and so you can't easily just get rid of their router... so there are cases where you can't, but I suspect if you have a GPON ONT or something similar you can just connect your RPi router direct to it)
Thatās exactly how I had it setup. 
I just removed it while testing with Bell support. That way they couldnāt try blaming my gear for my performance issues.
Normal setup:
Fibre => media converter => managed switch =>
Managed switch
Port 1: media converter
Port 2: Bell Modem/router WAN (for iptv)
Port 3: Rpi4 WAN (internet)
Port 4: Management for managed switch
Port 5: testlan (to test Rpi4 wan/lan throughput)
I once tried to get my Archer C7 to run both my iptv and internet and it never worked properly. My tv video kept on freezing every few seconds. So I gave up and got the managed switch so I could keep both going separately.
Hmmm... i don't remember the details but make sure the bell device can't talk to the VLAN where the RPi4 accessed the internet. If two routers fight over who routes what it could cause issues
I would change VLAN 35 to have only port 1 tagged and port 3 untagged, remove port 2 tagged. Then the bell device only handles the IPTV stuff. right now the Bell and the RPi seem to be fighting with the media converter for control over the internet packets on VLAN 35
Iāll give that a try tomorrow and see what I get. (My kids and my wife will lose it if I take anything down right now lol)
I think it was setup that way originally as the Bell receivers needed internet for certain features/apps and services it can handle.. such as Netflix and YouTube. I donāt think itās needed anymore.
I might even attempt seeing if my Rpi4 could handle iptv and internet. 
If I decide to completely remove the Bell HH3k and managed switch...
Is it possible to tag multiple vlans on the rpi4 if I hook up my fibre (via the media converter) directly to it?
For Bell I need these vlans:
vlan 35: internet (pppoe)
vlan 36: iptv
My Rpi4 has 2 ports:
eth1 (USB3 Tp-Link UE300) WAN
eth0 (onboard nic) LAN
My guess is it wouldnāt work.. but figured Iād ask
Short answer: of course the PI can tag different vlans. Create an interface called IPTV using eth1.36 and change WAN to be eth1.35. make port 3 tagged for both.
But... Does the Pi need to be involved in the IPTV at all? What do you watch the IPTV on? Maybe it can be plugged into a port that's untagged for vlan 36 and that's all you need?
I understand for the WAN port.. but for the IPTV interface (eth1.36), do I set that PPPOE as well or static/dhcp? (this was the part I wasn't sure about)
EDIT: port 3 won't exist if I remove the managed switch and connect directly to the fibre (via the media converter)
I was going to try and eliminate the managed tp-link switch and bell router/modem completely and let the Rpi4 run everything. I have 2 BellTV receivers that need to be hooked up to the Bell network on vlan 36 in order to obtain an IP address/etc (10.x.x.x). (if that all makes sense lol)
Like I mentioned before.. I attempted all of this before with my Archer C7 and I could never manage to get the Bell TV receivers to work properly.. the signal kept on freezing after a few seconds.. (something funky going on with igmp snooping)
I would guess the RPi should just do dhcp on the iptv vlan. But you might try just bypassing the RPi entirely and let the TVs just bridge to vlan 36 direct. The igmp snooping in that switch should work fine. Maybe make sure you have the latest firmware in the switch
so you're recommending I keep the managed switch (TP-LINK TL-SG105E) in my setup?
bell fibre => MediaConverter (TP-LINK MC220L) => SmartSwitch (TP-LINK TL-SG105E) => Rpi4
This will end up being pretty much the same setup I already had.. minus the Bell Router/Modem that only handle the TV receivers. I was trying to see if there was a way I could remove the TP-LINK TL-SG105E.... but I guess it still makes sense to keep it... especially if I want to continue being able to test my Rpi4 wan/lan throughput with my testlan on port 5 (you helped me setup)
You have vlans so you really are going to need a smart switch one way or another. The question is what is the bell device doing for you? Seems like nothing much and it might be battling for packets on the internet vlan.
Set it up like this:
Port 1: tagged for vlan 35 and 36, connect media converter
Port 2: tagged for vlan 35, and 3. connect RPi4 WAN, make wan use eth1.35
Port 3: untagged or tagged for vlan 36 (depending on TV box requirements), connect iptv box. make pvid be 36
Port 4: as above connect second tv box
Port 5: untagged vlan 3 for testing, pvid is 3
See how it goes
I guess the BELL device doesn't really do much lol
Won't I lose the ability to manage my smartswitch if I remove Port 4 from my current setup? Port 4 is currently hooked up to my LAN so I can connect to it with the TP-LINK config software.
The Bell receivers are both wireless. They're both connecting wirelessly to a Bell wireless transmitter (VAP2500) to connect them to the network.
So could I do this:
Port 1: tagged for vlan 35 and 36, connect media converter
Port 2: untagged or tagged for vlan 36 (depending on TV box requirements), connect Bell WIRELESS TRANSMITTER, make pvid be 36
Port 3: tagged for vlan 35, and 3. connect RPi4 WAN, make wan use eth1.35
Port 4: management to LAN
Port 5: untagged vlan 3 for testing, pvid is 3
I switched Port 2 and 3 so I don't have to move the Rpi4 to another port from it's current setup.
this make sense?
(btw I REALLY appreciate you taking the time to help me with all of this!)
You can actually get rid of a special management port for the LAN by adding eth1.1 to your br-lan in openwrt and make port 3 a tagged member of vlan 1
Then your plan could be just like you said, except port 4 can be used for whatever you like.
Also note that these switches listen on all vlans for management packets. So... get yourself a password keeper that can generate strong passwords (like keepassxc which is what I use) and generate a 12 character random password for management of your switch. This will keep someone who hacks your ISPs TV vlan or something like that from accessing your switch config.
Great advice! I use Bitwarden for my password management. They seem similar 
So for this to work, I should add another interface (eth1.1) and bridge it with my LAN? Will this be a static ip interface?
As for vlan 1 on the switch, it doesn't seem like I can make any changes to VLAN1.
Here's what I have right now:
Ok, thinking about this further since vlan1 is special on these switches, let's make your regular LAN be on vlan 2 and the testlan on vlan3. Make vlan 1 be for management only, and only accessible as a special "WAN" interface through OpenWrt:
Feel free to reorder ports if I'm forgetting what they're for or who's plugged where:
Port 1: tagged vlan 1,35,36 attach media converter
Port 2: TAGGED for vlan 1 (to limit access) and tagged/untagged as needs be for vlan 36, pvid is 36, plug Bell Wireless transmitter
Port 3: tagged for VLAN 1, 2, 3, 35 pvid 2. On openwrt Configure eth1.1 as a new interface "switchmgmt" add the eth1.2 as a physical member of your existing LAN bridge (make LAN a bridge across eth0 and eth1.2). keep eth1.3 as physical member of testlan interface.
Port 4: untagged member of VLAN 2 with PVID 2. This is a port you can use to access your LAN
Port 5: untagged member of VLAN 3 with PVID 3. This is a port you can use to access your testlan.
Make the switchmgmt interface use a different subnet from your LAN, and configure the switch to use a static IP in that subnet, for example 192.168.13.1 on openwrt and 192.168.13.2 for the switch.
Allow forwarding between LAN and switchmgmt. disallow forwarding from switchmgmt to anywhere.
Hopefully this makes sense?
I'll need a moment to decode all of this lol ... I keep on getting mixed up with PORTS and VLANS.. and I guess since I made my vlans 35 adn 36 can be a bit confusing too..
I have to picture it in my mind while looking at the VLAN interface on the managed SWITCH.
This is what I've decoded so far.. does this seem right?
VLAN 2 (switchmgmt): tagged ports: 3 | untagged port: 4 | Port 4: PVID: 2)
VLAN 3 (testlan): tagged ports: 3 | untagged port: 5 | Port 5: PVID: 3)
VLAN 35 (internet): tagged ports: 1 | untagged port: 3 | (Port 3: PVID: 35)
VLNA 36 (iptv wireless transmitter): tagged ports: 1 | untagged port: 2 | (Port 2: PVID:36)
Port Assignment:
Port 1: media converter (fibre)
Port 2: Bell TV wireless transmitters
Port 3: Rpi4 (OpenWRT) eth1 WAN (PPPoE)
Port 4: switchmgmt LAN? | OpenWRT eth1.2 LAN (Static: 192.168.13.1)
Port 5: testlan | OpenWRT eth1.3 LAN (Static: 192.168.20.1)
I'll figure out the OpenWRT interfaces once I get the switch side config ok 
Yeah, I could see how it'd be confusing because I was thinking ports, and the interface does one vlan at a time. I'll use the shorthand 1 for an untagged port 1t for tagged, kinda like in OpenWrt:
VLANS:
VLAN 1: "1t 2t 3t 4t 5" (note: while configuring the vlans remain plugged to port 5)
VLAN 2: "3t 4"
VLAN 3: "3t 5"
VLAN 35: "1t 3t"
VLAN 36: "1t 2t" (or 2 depending on bell wireless expectation)
PVIDs for each port:
1: 35
2: 36
3: 2
4: 2
5: 3 (once you configure this on a normal switch port 5 will no longer talk to the management interface on vlan 1, but these switches will listen on any vlan technically)
Openwrt interfaces:
lan: bridge: eth0 eth1.2
testlan: eth1.3
wan: eth1.35
switchmgmt: eth1.1
AVOIDING LOCKOUT:
my advice is connect via port 5 and leave port 5 untagged for vlan 1 until the very last thing you do is change its PVID to 3.