RT-AC87U Wifi drops internet connection randomly them reconnects

Have a really irritating issue with my router. I'll be using my phone or tablet and get a message " Internet is not available with (SSID). The WiFi does not drop, signal is strong, and a few seconds later the page will load - most of the time. Happens on both bands with private SSID or Guest. Have rebooted any number of times, updated from original to Merlin firmware, problem persists.

All my hard wired connections are fine, never drop internet.

Is there a solution I'm missing?...

RT-AC87U Wifi drops internet connection randomly them reconnects