I hope it will be useful.
After a sudden update to the BigSur 11.1 I faced an unexpected issue with WiFi connectivity, MacBook Pro 2016 (2019) declined to connect to my 5Ghz network, choosing only 2,4Ghz.
After many attempts I found out that there is a specific (at least in my case) parameters needed to be enabled in order to fix this.
I use TimeCapsule (Ops version - 7.8.1) as my router and in "Advance Wi-Fi settings" you should choose the following:
Radio mode: 802.11a/n-802.11b/g/n (automatic)Ghz Channel: Automatic
5Ghz Channel: 48
With these parameters turned on my Mac finally opened his cyber eyes to 5Ghz network.
Dmitry005
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Moscow, Central Federal District, Russia