Thanks @xerces8 for your finding.
This is a new lead to use for troubleshooting the problem.
I have been using OpeWRT for over 6-7 months now without a single problem. This problem started to happen 2 weeks ago. However, many people here are complaining that the ISPs are pushing to sell their routers when subscribing to new xDSL services. Perhaps the ISPs are changing their polices.
For the matter of this: I tried to clone my ISP modem MAC address and use it in OpenWRT but I couldn't connect.
It may help but has the caveat of disabling media sensing on the (WAN) port.
Now I am two months later testing. The problem seems to be with OpenWRT. Returning back to the stock firmware running on Partition 1, the problem doesn't happen. Running a TPLink Archer VR600 doesn't cause this issue. It is only happening with OpenWRT. I upgraded my OpenWRT image to 19.07.3, and the problem is still happening.
I haven't tried it, but I don't want to lose the WAN media sensing too. I think this will disable the ifdown state of the WAN Port. Right?
Maybe this is the case? How to hide that I am using a custom router? How ISP actually senses that? Through my MAC Address? If so, Isn't MAC Cloning possible to masquerade my custom router's visibility?
EDIT:
@xerces8, I have noticed that you have worked out a solution, from your thread, to the PADT being sent by the AC and processed by the router. May I get a link to the patch file? I am running on 19.07.3.
Thanks @xerces8 for your response. I believe, I will have to edit the pppoe.c in the source code manually then compile and build instead of patching using quilt. Right?
I used quilt.
Otherwise the build system might overwrite your changes. It is very complex. Ask someone who knows it.
What I did was: checkout the sources from git, create a "proper" patch with quilt (follow the instructions from the wiki*, "quilt new" , then apply the patch, manually, or simply edit the file and add those two lines,
then do "quilt refresh", make update and the full make - note: this is a quick description, just to give you the rough idea, not all steps are listed)
* https://openwrt.org/docs/guide-developer/build-system/use-patches-with-buildsystem
I also have a WRT1900ACS router, using a Netgear DGND3700v2 device as the ADSL2+ modem to connect to our provider.
The Netgear device is setup on it's own IP: 192.168.0.1 , subnet 255.255.255.0 .
DHCP is disabled, all options are at factory default.
A Cat5e LAN cable is connected to one of the Netgear's LAN ports, and is connected to the Linksys router's WAN port.
The Linksys WRT1900ACSv2 router WAN interface is set as:
Protocol = Static Address,
Bring up on boot = checked / enabled,
IPV4 Address: 192.168.0.2,
IPV4 Netmask: 255.255.255.0,
IPV4 Gateway: 192.168.0.1
I have found this is the most stable configuration for our setup here. Allowing the "provider" (PPOe) modem to handle negotiation <-> communication with the Provider. As we currently do not have any IPTV, etc. setup, there are no "Double NAT" issues .. with this configuration.
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I know this does not address any (PPOe) config issues you are currently experiencing: but thought the info may be useful to you in aiding any troubleshooting / further isolating the issue.