I discovered the same issue.
Whenever I get out of reach from a WIFI, when coming back into reach the FP2 does log on again, but uses the internet connection from the SIM instead of the WIFI and showing an exclamation mark on the WIFI symbol.
So far I bypassed the problem by deleting the WIFI connection from the known connections every time after coming back, and newly logged in again (incl. entering password again; which, I think I don't need to stress, is quite annoying).
When returning from flightmode on turning off/on the phone, there was no problem with getting on the WIFI again as supposed, only when getting out of reach.
Do you mean you changed the frequency selection once and the problem was solved permanently, or do you have to change the frequency band then every time you come back to the WIFI reach?