I got it working by changing the port used by the Wireguard vpn on the GL.iNet device.
I'm thinking that port 51820 is reserved for the ASUS router's own Wireguard VPN server and it won't let you direct it to another IP…although I can't find any documentation to confirm that.
I just got an ASUS RT-AX58U to replace an old Linksys router that was running an old version of DD-WRT. Connected to the old router via ethernet was a GL.iNet travel router on which I had a Wireguard VPN server working perfectly. I was hoping I could just set up the new ASUS router (done), add...