Yes, use “Connect a single device” for GL-MT3000.
Most likely the endpoint address and port “xxxxxxxxxxxxxxxxxxxx.myfritz.net:xxxxx” is not accessible in your GL-MT3000 network.
run command:
nslookup xxxxxxxxxxxxxxxxxxxx.myfritz.net:
To tell if it’s resolved to your FRITZ!Box’s public IP.
If it doesn’t have a public IP, you need to setup port forward on the modem or upstream router.