I am back at my parents place and tried again to replace the existing TP-Link with OpenWrt with the Asus RT-AX53U (23.05.2), but after a restart WAN still fails to come up again.
I will upgrade to 23.05.3 now and see if there is any difference.
Edit: upgraded to 23.05.3 and it is still the same.
This is a part from the log:
Mon Mar 25 12:57:02 2024 kern.info kernel: [ 73.551268] mtk_soc_eth 1e100000.ethernet wan: Link is Up - 100Mbps/Full - flow control off
Mon Mar 25 12:57:02 2024 kern.info kernel: [ 73.559712] IPv6: ADDRCONF(NETDEV_CHANGE): wan: link becomes ready
Mon Mar 25 12:57:02 2024 daemon.notice netifd: Network device 'wan' link is up
Mon Mar 25 12:57:02 2024 daemon.notice netifd: Interface 'wan' has link connectivity
Mon Mar 25 12:57:02 2024 daemon.notice netifd: Interface 'wan' is setting up now
Mon Mar 25 12:57:02 2024 daemon.notice netifd: Interface 'wan6' has link connectivity
Mon Mar 25 12:57:02 2024 daemon.notice netifd: Interface 'wan6' is setting up now
Mon Mar 25 12:57:02 2024 daemon.notice netifd: wan (4604): udhcpc: started, v1.36.1
Mon Mar 25 12:57:02 2024 daemon.err odhcp6c[4605]: Failed to send RS (Address not available)
Mon Mar 25 12:57:02 2024 daemon.err odhcp6c[4605]: Failed to send SOLICIT message to ff02::1:2 (Address not available)
Mon Mar 25 12:57:02 2024 daemon.notice netifd: wan (4604): udhcpc: broadcasting discover
Mon Mar 25 12:57:03 2024 kern.info kernel: [ 74.591401] mtk_soc_eth 1e100000.ethernet wan: Link is Down
Mon Mar 25 12:57:03 2024 daemon.notice netifd: Network device 'wan' link is down
Mon Mar 25 12:57:03 2024 daemon.notice netifd: Interface 'wan' has link connectivity loss
Mon Mar 25 12:57:03 2024 daemon.err odhcp6c[4605]: Failed to send RS (Address not available)
Mon Mar 25 12:57:03 2024 daemon.notice netifd: Interface 'wan6' has link connectivity loss
Mon Mar 25 12:57:03 2024 daemon.notice netifd: wan (4604): udhcpc: received SIGTERM
Mon Mar 25 12:57:03 2024 daemon.notice netifd: wan (4604): udhcpc: entering released state
Mon Mar 25 12:57:03 2024 daemon.notice netifd: wan (4604): Command failed: ubus call network.interface notify_proto { "action": 0, "link-up": false, "keep": false, "interface": "wan" } (Permission denied)
Mon Mar 25 12:57:03 2024 daemon.notice netifd: Interface 'wan' is now down
Mon Mar 25 12:57:03 2024 daemon.notice netifd: Interface 'wan6' is now down
When I connect a switch between the wall plug and the router, WAN comes up.
The wall plug is connected with an old cable (hence only 100 MBit/s) to the upstream router. As however both the TP-Link and the switch have no issues establishing a connection, I doubt the cable to be the problem here.