Connect Imac via Ethernet Cable

Hi!

Can someone please help me. My nighthawk M1 Mobile router works fine via wifi but as soon as I connect via an ethernet cable to my 2015 iMac (and dissconnect wifi), than it stops transfering data. It doenst work annymore. It did work fine yesterday.

It some does work when I connect it to my Dell notebook via an ethernet cable. But I would like to use it as well on my iMac

many thanks for your help here!

Pat.