Wifi connection issue

I have a router and airport base station (express). I access either from mac desk top, iphone and ipad.

Recent problems have come up.

The wifi drop down menu acknowledges the router and the base station. Also shows connectivity. But, internet does not work.

The AirPort Utlity does not acknowledge, saying the router and base station are disconnected.

One or more of the devices will sometimes connect and work, then fail to work days later.

Relatedly, the base station now blinks yellow, after I tried using it for extended range, not knowing what I was doing, and then reconnecting it as a base station. Even though it blinks yellow, I was still able to use it successfully at times.