Port forwarding getting lost

I'm having a bit of trouble setting up an LB2120 4G router and wonder if anyone can help please? The latest firmware is loaded...

I have got a public ip address for the router with the mobile network provider.

The LB2120 is in Router mode and I've given it a static ip address to match our internal network numbering, and disabled its DHCP service.

If I connect the LB2120 directly to a laptop on the same network and use the LB2120's address as the laptop's default gateway I am able to access the Internet. All good so far...

I set up a port forward rule on the LB2120 to forward ssh traffic on port 22 to the internal address of my laptop which is running OpenSSH server.

From another (external) device I am able to connect to the SSH server on the laptop via the public ip address of the LB2120. Still so far so good...

The problem comes when I connect the LB2120 to a switch on our internal network and try to connect to, for example, an ssh service running on another server within our internal network. I modify the port forwarding rule on the LB2120 so that it goes to the address of the other server (rather than the laptop's address). Save the rule, restart the LB2120.

When I now try to connect from outside our LAN to the ssh port via the LB2120's public ip address, it fails to connect and eventually times out.

Am I doing something wrong in the setup? Should I be able to connect the LB2120 to a switch on our internal network and expect the traffic coming in from it's public ip address to find it's way to the correct internal address and port (22 in this case) and back out again to the external host? The traffic seems to be getting lost somewhere along the way when I connect the LB2120 to our internet switch.

Any bright ideas where things could be going wrong? Could something along the path not now how to route the traffic back to the external host via the LB2120?

Thanks for any advice...