Hi and thanks all for your feedback.
I'm trying to avoid bridge mode because I still want to be able to administrate the Connect box and also because en(dias)abling bridge mode requires customer care support. Also I'd like to make use of the free Ethernet plugs available on the Connect box.
I made several attemps in disablig DHCP but that didn't end well. I was unbable anymore to connect to the Connect Box, my laptop connected to the Connect Box via ethernet was getting an IPv6 address and I was not even able to even ping 220.127.116.11 (not sure why) so in the end I had to reset the Connect Box.
What I tried as well was to enable DHCP from 192.168.178.50 and assign a reserved IP for the devices plugged on the Ethernet ports out of the DHCP range (based on their MAC address) but it looks like those “reserved” IPs rules are not used anymore when DHCP is deactivated so it didn't work in the end…
My router is an Asus AX 11000 and it has quite some flexibility in terms of port forwarding, NAT configuration and Routing, but isn't it the Connext Box that needs to be configured in that case? Because I would imagine that what I need is to have the devices connected to the Connect box to “broadcast” their IP/Route to the other subnetwork so they can be visible….
But again I'm not sure…
Thanks again for your help