Hi xuggs, I’m trying to do the same as you.
My main router is a Fritzbox 7590 (with public IP) and is connected via LAN-WAN to the Gl.inet.
How did you solve the issue you had accessing the Gl.inet router connected to you main one ?

I’ve already setup port forwarding from the Fritzbox to the Gl.inet router for ports 80 and 443. Still no success when typing the local address of the Gl.inet router (192.168.187.1), nor via the ddns address unfortunately.