I set up one of these routers many months ago and had similar problems. If you go into the advanced settings in the router, you will find the priority settings for wifi & ethernet.
In the end I set them up as equal.
Another little quirk that showed itself later was, I set the router up on a sunday and the wifi connected computer could only get onto the internet on a Sunday so watch the days of the week settings.
router address is http: //192.168.1.1 (without the spaces after the
and the default password is 1234 unless you have changed it
Max