This is extremely discouraging news because it sounds like there's a possibility that this could be a hardware design issue. If that is the case, no software update can resolve it. I sincerely hope that is not the case!I went to the Apple store today (BTW the staff were excellent) and provided detailed iPhone XS Max stats comparing my wifi coverage in my basement versus the home office (where my router is located).
In the basement wifi speeds and signal strength dropped to 0 for the 5Ghz band for my iPhone XS Max while my other Apple devices (iPhone 7+ and iPad Pro) had no problem getting strong signal strength from my Linksys WRT1900AC router's 5Ghz signal and speeds were 80mbps+ (not as good as in my office but entirely OK for me).
The XS could not detect the 5Ghz signal in the basement and had no problem with the 2.4 band although speeds were <5mps (not acceptable for me).
In my office the XS wifi speeds were excellent and equivalent to my other Apple devices.
Apple generously swapped my phone to the same model. I used that phone as soon as I got home with only the Speedtest app installed and unfortunately I had almost exactly poor results.
I hope a software update fixes this issue or I will return the XS phone and keep my trusted 7+. I have tried all the posted possible solutions for this issue.
The other XS features are excellent compared to my 7+.