Why are multiple devices (security cameras, smart plugs, smart bulbs, etc) keep going offline???

Multiple devices are offline suddenly for the past few days.  I have not installed any new router or devices. I've tried resetting the router, the devices, and unplugged and plugged both to see.  I've delete the devices and re-installed them.  After router reset, it might function for a short period and then go offline again.  The wifi signal is strong.  One device, if I open and stay on it long enough, it might reconnect again.  So frustrating!!