I saved the config from my online router and restored it on another device.
The difference is that LAN iface is i218 instead of i210 and WAN is Linksys USB instead of RT8101.
In the text console, I assigned interfaces, assigned a static IPv4 and mask to the LAN iface, and went to the web GUI to provide PPPoE credentials. So far all seemed to be fine.
When I swap the routers the new one does not work because it has no gateway for WAN PPPOE. I tried to create it manually, but apparently I am doing it wrong because it stays defunct/down/disabled, and there seems to be no way to enable it even after WAN PPPoE comes up.
I do not recall having to create a gateway manually. If I remember correctly it was automatically created when I assigned PPPoE to the WAN iface. I went through most of the admin web UI and compared them side by side but could not find a difference.
What am I doing wrong with the new device?
There was a topic about the exactly same issue a while ago but without replies: https://forum.opnsense.org/index.php?topic=17551.0
Is this a taboo topic?
I used to have a setup where a virtualised OPNsense was on standby for one running physical hardware. I used to download the config XML from one, pipe it through sed to replace the interface names and then restore it on the other. It's been a while since I last did that, but it worked flawlessly every time.
I always transferred configurations this way because renaming interfaces on the console was too error prone for me. Other than a typo, maybe there's something in your configuration that's referring to interface names that doesn't get fixed by reassigning them?