If you just want to access the GL.iNet router Admin Panel from the Fritz LAN, then add port forwarding rule on the GL.iNet router WAN TCP Port 80 to LAN Port 80 at IP address 192.168.7.1, via Admin Panel → Network → Firewall → Port Forwards. I suggest also adding TCP port 22 for SSH access.
The routes on the Fritz will not get through the GL.iNet router firewall.
I do not work for and I am not directly associated with GL.iNet
It sounds like you just need to open port 80 or 443 on the MT300N’s firewall page. Then access the MT300N directly with 192.167.1.125 when the client connects to fritz.