If the PLL doesn’t lock (or doesn’t lock reliably), you have no way of knowing what frequency you are receiving.
I’ve never had the pleasure of FC0012, so no idea what’s inside the dongle, but those loaded with the 820 are very sensitive to low voltage supply. Try a ‘better’ PSU and see if you can get the tuner to lock.
Maybe.
The 820T datasheet claims 42MHz to 1002MHz, but obviously we can squeeze more that out of them.
Perhaps the FC0012 will surprise us.