Problem solved by my 10 year old son. (I've owned Macintosh computers since 1987, and should have known to do this; it's something I've made sure of in the past.):
Cable from SURFboard modem to AirPort Base Station was plugged into the LAN port rather than the WAN port.
For over two months I'd...
I can connect only two devices (maximum) to my wifi and/or internet simultaneously via my home network. This limit first started happening two months ago, in August. Connecting a third device requires restarting the modem & router, but one of the other two devices is removed from the network...