If the Glinet router works as a VPN server and it is behind an upstream ( Main Router) in your case Fritz Box 7590, that shares the Internet to Glinet devices, you need to allow in your benutzeroberfläche von Fritzbox (Internet > Freigaben > Neue Freigabe ) the port (in your case standart Wireguard UDP port 51820 ) for the IP address of your Glinet router AXT1800. If you still need help after these manipulations, you can send me a private message. If your language is German, I will be able to answer your questions more correctly. 
I have an AXT1800 as a Wireguard Server behind Frizbox, it works perfect.
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