Last night, just after midnight, my Tapo app reported "network unstable".  I checked again this morning and found the same result.
My router tests at a reliable download speed in excess of 100 Mbs and further attempts to connect to the cameras, even standing close to both router and cameras, returns the same network unstable message.  I tried turning off the power to both cameras, then back on again with exactly the same result.

The strange thing is that I receive messages of movements from the cameras, but cannot connect to them.

My two Ring cameras, on the other hand, are working faultlessly.
I have followed the published suggestions about rebooting the router, etc., but this has achieved nothing.

I assume, then, that the fault may lie within the app on my smart phone.
I should be very grateful for any advice.