R7900P ethernet ports not working when i connect a second ethernet cable

[Hotdon]

I have a strange issue where when I connect a second ethernet cable to my router, the Lan ports lose internet connection but the wifi still works. I tried rebooting both modem and router, factory resetting the router and even bridge mode. None seem to fix the issue. The only way that I could get both ports to be detected was by disabling DHCP.
The second ethernet connection comes from a unmanaged switch where I have connected a POS computer and a kitchen printer. And the router is not assigning an ip address to the pos computer.

I am out of ideas, please help.

[plemans]

Does it happen if the switch isn't use? 

Or if nothing is plugged into the switch? 

Just curious if either the switch or the POS is causing a loop issue. 

[Hotdon]

I have not tested it without the switch. I will test it later on the week and post back results.