I had the same problem and I think I found the solution,
first of all I tried rebooting and resetting my Nokia 8 and I didn't fix it, updateing the phone did not fix it, returning the phone for a replacement I found that the new phone showed the 5ghz band then an error message appeared and no 5ghz wifi were shown anymore!
the problem is when you set up a new phone you will choose a country, this option will set not only the language but other settings as well among them the channels used by 5ghz wifi in your country! the router might be set to a different country using different channel!
solution: access your router settings and check your wifi 5ghz advanced settings choose a country matching your phone settings and restart your router.
if this didn't work try an android app called wifi analyzer and check channel settings for 5ghz after choosing your country it will show you which channel are open for 5ghz in your country,pick a channel and return to your router 5ghz wlan advanced settings and change the channel option from automatic to your the one from wifi analyzer and apply and restart your router!
hopefully this will work