TP-Link Archer C2 - bidge WAN+LAN (almost working)

Hi. This is my first experience with OpenWrt, so I'm learning as I go.

I guess I need help with configuration/troubleshooting a network issue.

I've used a double-NAT setup for a while now, primary router (Linksys wrt54g with DD-WRT firmware). Secondary router is the Archer C2, who's WAN port is connected to linksys LAN, and obatins IP address via DHCP. Static routing configured on linksys so all devices can talk to each other. All is good.

Current Goal
I want to use a Raspberry Pi (with pi-hole) as DNS / DHCP server for the entire network. For this I should configure the Archer C2 to act like a switch, ie. disable NAT & DHCP. Clients will then access the upstream DHCP? Is this correct?

I'll mention that due to the number of devices, I need to use all 5 ports on the Archer C2. There is no option for bridge mode in stock firmware, so I flashed OpenWrt firmware:

To be continued...
(seems as a new user, I can only include one image per post)...


Bridge WAN + LAN:

Under LAN interfaces section, I added eth0 and eth0.2
Also changed protocol from static to dhcp client
Somewhere there was a dhcp server option "ignore for this interface", which I can no longer find

There were also interfaces for WAN and WAN6, which I had initially set to "unspecified". However this resulted in an error loading the web gui, and I had to ssh in and comment them out in /etc/config/network

To be continued...
(seems as a new user, I can only include one image per post)...

(continued from previous post)

This is the current VLAN configuration (these are all the default settings, I haven't made any changes here):

I'm really not clear on how this vlan configuration works, or what is meant by tagged/untagged, which is why I've left this alone.

At the moment the WAN port is unused, I have devices plugged into the LAN ports of Archer C2 only.

The problem
DHCP is ok - I can see main router is issuing IP address to clients connected to Archer C2 and all devices have internet connectivity. Some devices cannot see each other however. Testing shows LAN clients can connect to wireless clients, and vice versa, but the LAN clients cannot see each other. When I try to ping them the error is "Destination host unreachable".

In particular, this means I can no longer access the OpenWrt web configuration from the LAN side of the Archer C2. I can only access it via wireless connection, and I don't know why.

I'm not sure what to do next, or even if my configuration is correct, and this is about the extent of my networking knowledge.

Happy to post config files or other details if needed, this post is rather long so I'll leave it for now.

-- Stay safe, everyone!