Network settings revert to yellow, no connection

Confused and befuddled:

Problem began a few months ago. We have two macs both on wireless and also my iphone. One of our computers would get disconnected from internet (the iphone kept working). I would pull the plug on the modem and router to reset and that would fix it. Resetting the router alone was not sufficient, both the router and the modem had to be reset. But began happening a few times every day. Sometimes would occur when only one of the computers was on. Then a few weeks ago, malfunction disappeard and both computers stayed connected for two weeks with no problems. But last week had a guest with a PC laptop stay for a week and problem came roaring back. I was constantly resetting so that one of the computers could stay on internet. Guest with PC has left but problem persists. I was never clear on the pattern. Certainly if only one computer is on, resetting the devices, will get that computer back on. But with more than one computer, sometimes more than one computer would work, sometimes not. The typical diagnositc status is the wi-fi and wi-fi setting buttons are green, the network setting is yellow, the internet and server are red, sometimes the isp is green, sometimes red.

Thank you