I don't have anyone around me using 5G band other than my wireless phones at home. So I think that that's part of the problem. I wonder the phones at what or close to what channel they are transmitting.
that explains things. 5.8GHz phone and 5GHz wifi - least interference using lower channels. I use DECT 1.9 GHz phones here.
so I see why you have issues with the upper bands,
It would be interesting to see if others with issues also have 5.8Ghz cordless phones.