hello,
the fritzbox is acting as a router and doing NAT.
you would need to create a firewall rule, on the fritzbox, to forward the ports from the fritzbox to the ar750.
on the fritzbox, it is called port sharing
also, it might be a problem as the fritzbox might be using 443 and 80 for itself.
so better to create port translation rule.
forward port 444 on the fritzbox to port 443 on the ip address of ar750.
then you would access the at750 as https://81.164.xxx.xxx:444
not sure if the fritzbox can do that.
if not, on the ar750, change the port for the webgui to 444, then setup a port sharing rule in the fritzbox
for port 444 to the ar750
for what it worth, not a good idea of expose a router’s web interface to the entire internet.
better to use a vpn or ssh.