Evening!
How frustrating!!! As the person who posted about connecting directly to a router I feel somewhat responsible!
Maybe try resetting the handset before then trying to register:
If a Mk1 (no INT button) press (dot) 7 000 9
If a Mk2 (with INT button) the Service Menu: 9#9#9*INT allows you to reset the handset.
Otherwise, try a reset followed by leaving the handset on the charging pyramid base. (Logically a PIN should only be required if the handset and PSTN are physically separated, hence worth a try!)