Modem Mode for Hub5x

Yes but its not a feature it more of a needed thing for the internet VM seem to forget years ago that how the internet was just a modem with a WAN IP and they just think double NAT is fine when its not and some routers with DDNS support might not work like that because they expect the WAN to have a WAN IP to update the DDNS.

So in the mean time just double NAT with DMZ to your setup some ports might be blocked like 25 and 22 and maybe some others.