So you have a single mobile device connecting via wg, correct? Not a second network?

If so the only configuration you should need on the router are firewall rules to allow forwarding from wg to lan. I am guessing that’s what the second step in the GL-iNet help accomplishes. That’s what I would recommend trying.

If I was doing this on “normal” OpenWRT I would create a firewall zone wg, and allow traffic to/from it and lan.

On the client device you should be good to go without changes if AllowedIPs is set to 0.0.0.0/, ::/0 (sending all traffic through wg).