that is strange, not to allow you to change the dest port.
there should be a easy way to change the web port on the ar750 from 443 to 444.
tho that is not safe to do, to expose a web interface of a router over the interent.
i guess you do not want to use vpn or ssh?
i have setup 100+ routers that sit behind another router, like in your case.
in my case, i need to use a certain router for voip service and double nat creates many problems.
for the front router, in this case, the fritzbox, i dumb it down, disable the firewall, nat, dhcp, etc.
basically turn the router into a modem.