mecool m8s pro + ethernet

i purchased for a friend a m8s pro + and as soon as the ethernet cable is plugged in. the stream will play for 3 seconds and then stop.. as soon as i unplug the cable and connect to wifi it plays ok.. but buffers a little bit. ( as the router is not very close to the box). have tried different cables but still the same also tired at my home with different router and modem but still the same.
any ideas on the problem... if its the port in the box thats at fault can a usb to RJ45 adapter be used so he can have Ethernet capabilities... if so can anyone recommend an adapter to use
thanks in advance