OK, you asked for it.
I have a 2-level house which includes 3 laptops, 3 mobile phones a wifi printer, a smart TV... and a 2013 iMac which is my workhorse. My network coped—just—using the Hwawei modem/router, but there were areas of the house with weak signals. So to improve coverage I bought the Google Nest Wifi Mesh and attached it to my modem. Everything linked up fine, powerful signal, I set up new network name and signed everybody in. Fine, except my iMac would not recognise the new network.
Here's what I tried:
—turned Wifi off and on (iMac)
—I re-booted the modem
—I re-booted the Google Mesh router
—I re-started the iMac (several times)
—I removed the new network from my iCloud Keychain and signed in again... and again.
—I added a public DNS to my server list (208.67.222.220)
Can't remember any more. Nothing worked.
THEN I noticed that the previous network was also available—still being kicked out by the Hwawei modem/router—and discovered my iMac would still recognise it. So now I have a house where 9/10 computers are in the Mesh, but my iMac is on the old network.
It works, but the problem is I can't print from the iMac. Aaarrrggg!