Did you try a simple factory reset

Not yet.  Rebooted, powered down, to no avail.  Would my ISP need to re-provision if I reset?

Well, that seems to have worked.  For now.  It seems that if I make any change in the settings (like a channel or rename) the Modem/Router has to be rebooted.

And away it went.  Had connectivity for a few minutes, then it’s off again.  aaaaaaaaaaaaarhg

How old is the device? 

what happens if you change the channel for the wireless when its not working?

Thanks to those who tried to help.  Well, we can close this one -- unless someone has a fix for my PC.  Now, after some fiddling with settings, my laptop, phones, 2 other PCs all connect OK.  It’s down to just one PC, and that one is about 12 years old, and has recently displayed the Blue Screen Of Death a few times.  Oh, well, time for a replacement!  

PC was about 12.  Changing channel no help.