Dear @rp201rp,
first of all thanks for your reply.
I’ve been working a bit on your notes and have to say that I’e implemented the port-forwarding as you suggested. Unfortunately I’m still not getting what would like to achieved.
My main goal is to access via WiFi the configuration page of my GL-AR750 (to then activate the ExpressVPN), being still connected to my Fritzbox network.
As said in my initial post, I’ve been following this video-guide in order to link the 2 routers.
I then assigned the following IP addresses and IP pools as in picture below.
But I soon realized that if I use the WAN port as LAN to connect the AR750 to the FritzBox, I cannot use the OpenVPN function.
On the other hand, if I use the WAN port, I cannot access the AR750 anymore since it’s receiving one address (192.168.188.21) from the FritzBox which is different from the one I’ve assigned to access it (192.168.188.2).
Feels a bit like in a maze 
Any recommendation on what I’m doing wrong ?
Thanks a lot for your support !
