Using your guide, I was able to change the product code and region code to 000-88, thank you. It looks like with this changes wifi at last works stable on channels 149+ (the phone has detected them previously, but couldn't use properly - I had no internet after connecting to them; I think this limitation was due to EU version of the phone). However, it didn't enable such an options as reset protection or manual wifi address setting, which is a bit strange. Are these features available on your phone?
Thanks anyway, as the 5G wifi problem kinda annoyed me.
Edit: Nope, this didn't help - still no internet on AC connection on channels 149+