BUT one device (travel router) does not work via wireguard in one specific lan
What LAN is this? At home? At a remote site? where?
What is the ip/subnet of said LAN? Does it overlap with anything at your home?
my travel router (gl.inet Opal) though connects with wireguard but effectively almost no traffic passes through. I can contact my home network and can ssh into the raspberry etc... but no WAN connection and no larger data chunks eg via plex
Is this on a particular network that it doesnt work like a hotel or location? You said you tested the travel router at home using mobile hotspot and it worked so what network did it not work on?