5060 inbound blocked / can't voip anymore with Zyxel router

Hi,

just installed the Zyxell modem; turned off the firewall features; 

unticked the DHCP option 125

This is the sole & main router in the network

All my voip phones continue to work but my work phone; which I guess is more "secured"  (not too sure but I think its tls 1.2) 

It simply wont register  (its got an IP just fine, I can ping 8.8.8.8 with the built in tool, etc.)

If i replace the router with my good old DIR-825 running DD-WRT;

all is well.

Problem is my dir825 is quite old, and wanted to leverage better Wifi performance of the Zyxell.

Any ideas ?