iMac 27" 2020 Sonoma 14.4.1 Wifi will not connect or hold a connection

We have had Wifi troubles with this mac for a long time. After a power cut today it refuses to connect to our home network. We have power-cycled the iMac and power-cycled the wifi modem several times, but it keeps failing to connect. It pops a 'connection timeout' screen repeatedly, asking us to re-type the password each time. All other devices in the house are happily connected (Mac Mini, iPad, two phones, Netflix), so we know the password and the network is up and running fine. Telling the Mac to "Forget this network", then re-creating the network and trying to connect doesn't work either.

For what it's worth, when similar problems appeared several years ago, a genius bar tech showed us how to solve the problem by creating a new "location" in the System Preferences. System Preferences have now been replaced by "Settings", and I can't figure out how to perform a similar procedure.