@GiuseppeC  Hello, 

When  the gateway router connect to the WAN port of MR600, MR600 should get Internet directly working as a Router. 

And most of the advanced features including oneMesh, 3G/4Ghz failover function only work when MR600 works in Router Mode. 

To route the traffic, if you want all the devices to work in the same LAN (192.168.0.X), you have to turn off MR600 DHCP function but it will lose the advanced settings I mentioned above. This is the same for most routers, all advanced feature relying WAN to work properly. 

If you only want to access a Server behind MR600 network, you could open port for MR600 on the gateway router, and open port for the Server on MR600, as there is double NAT in your network. 

 PFSENSE (with NAT)

198.168.0.1 gateway , internal network: 192.168.0. ----<Wired>-- MR600 ( work as a NAT device, using a different subnet like 192.168.5.X)

|         |    |

|         |    |RE220

|         |RE220

RE220