PPPoE does not connect after PADS

hi I am using OpenWRT on an x86 machine and the connection between me and my ISP is VDSL using PPPoE without any VLANs required

I have 2 modems ZTE 168n and tp-link 9960. I am currently using the tp-link as my main modem because the zte one has a very severe issue with line speed.

the problem is if the line is disconnected for more than 5 minutes I can't connect back to the internet (in the interfaces tab it says negotiation failed) until I use the zte modem. then I can connect the tp-link back without any issues. so I took a packet capture. when the tp-link is connected every stage of PPoE seemed to happen correctly PADI->PADO->PADR->PADS with lots of LCP packets in between. but after receiving the PADS nothing happens and the OpenWRT sends PADT and starts the cyclic from the start and enters a loop. the same thing happens with the ZTE modem but it will eventually connect after 4/5 minutes. I contacted my isp but they have no idea what is going on.

any help? I can post the PCAP files if anyone is interested

Kinda reminds me very closely of my old PPPoE issue with the BT HH5A.

I'm not sure if that was exactly the same packets exchange, looks like I stripped it from that post for some reason, and I can't even find the full PPPoE session logs I stored on disk now, but that was not an ISP issue because it always got fixed as soon as I rebooted, and also in many years of crappy commercial modems it never happened not even once on that same line... and similarly din't happen again when I rolled back to openwrt-as-a-router-only setup.

For me it was simply not a resilient solution in the situation of cables disconnections which could happen at home, but in the end it was not worth to investigate further given my use case.

But does the tp-link alone (as modem+router) have the same issue, or does it only happen when in bridge mode with PPPoE passthrough from the OpenWrt machine?

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the tp-link alone works fine but I noticed that it takes a lot of time (around 5 minutes) to get an IP from my ISP this also happens with every router I tried.

rebooting the tp-link does not fix it at all. I talked to some people with the same isp and some of them have the same issue with different modems. even someone posted a similar issue here in OpenWrt but the thread is dead.

For some reason, I don't think it is a modem issue because the router successfully start negotiating with my isp devices thus the bridge functionality works as expected. and I don't think it's OpenWrt fault either as i tried with OPNsense box and the same exact issue happens