I have nothing of inside information or support feedback - but stumbled upon the licensing certificate within the “About Phone” settings category.
I fear we can mark this “wont fix” (or someone checks if a FP3+ has a different wording), I quote:
“The 5GHz wireless LAN frequency band is restricted to indoor use only throughout the EU”
Having looked up what the indoor use bands / frequency areas are, I fear that this is correlated to the observations of connection problems above band 100, as the area of strictly indoor use is in the range below. Compare here i.e.
On that page is written in german the recommendation to use the lower frequency bands at the access point, as was pointed out as a workaround here earlier already.
Considering that there now is a workaround to reliably have the frequency chooser within the WiFi settings menu, I can just recommend to use the 2.4GHz only option if you fail to connect to a network where you don’t have administrative access.
BTW, my wificountrycode reads US too, and reading the licensing I’d expect that to be the least restrictive option already. Deducing it might be an hardware thing which motivated that licensing.