Big Sur Wifi only works online but not on local devices

>>if your wifi router does not bridge 2.4Ghz wifi 5Ghz wifi .....<<

Solution found.

After your massage I compared the three Macs WiFi windows by holding "contol key" while clicking on the wifi symbol in the top manu bar and found no evident difference between them so I deducted that a total DHCP reset was needed.

I turned all Mac off, the router off and waited a good 15minutes to make sure that all capacitors would de-energize before switching the router on again and, despite failure after previous attempts, this time the reset toke place and everything has gone back to normal on the one running on Big Sur.

It is possible that it chose a different channel during Big Sur installation (or the recovery attempting to fix the problem) as it was connected via Ethernet at the time.

Many thanks for confirming the 2.4 / 5.0 Ghz potential problem which induced me to investigate furhter

None of the above, just a fresh Big Sur install

thanks

That is what I was afraid but it always worked before and stil does with the older OS Catalina and OSX El Capitan on the other two Macs.

Is there a way to find out which frequency is my Mac connection is using perhaps?

Thanks

Sorry, I should have said I'm not aware of any unless they installed on their own accord when the upgrade toke place