NightHawk M1100 (M1) port routing issue

Hello, 

My topology is The M1(last firmware available) with external outdoor antenna and cellular web access wired to a Zywall 20 USG wired to a DS216 Synology NAS. (to make it simple).

Bandwidth is really nice and constant, the M1 does a really good job !

The Zywall has a fixed IP and is not part of the M1 dhcp range.

I run a VPN server on the NAS (port 123456 for example). I did all the routing from each WAN to each LAN of the M1 (via-router-port forwarding) and of course, the same on the Zywall so that the incoming connection:123456 reaches the NAS. The problem is that the M1 does not route.

Is there any trick I forgot ?

Thank you.