Hello MrMarkSH 

                             For no internet issue on WIFI, we need to check the light status on the modem, Since you have done with all possible trouble shooting no need to repeat again and also make sure you have properly reset modem without coax cable for 30secs, after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password to connect to your network.

                             If still no internet on WIFI you can try changing the WIFI credentials on the modem interface or GUI and connect your devices to the new network then check for internet.

Let us know if this helps.

Well, I had tried all that, to no avail.  Finally, I went to two other pc’s and an Android phone, and everything was fine from there.  So, having suffered through more than one BSOD on the 10+ year old, not-working-right pc, I bit the bullet and bought a new one.

Problem solved.