After upgrading to Oreo the my phone is now asking for a PIN for me to get in.

Samsung Galaxy A5

Offered to update software to Oreo

Took "Update now" option
Now asking me for 4 digit PIN No.
Have tried each of the only two I would have specified.
"Not known"

Looked on Internet and it says to reset PIN using Android Device Manager (ADM)
Logged into that and it can see the phone
There is no "Lock" option (apparently this allows you to reset the PIN?)
Took the "Secure Device" option - not invited to reset the PIN
Message "Lock (or something) requested"
Still can't log in to the phone.

ADM can still see the phone.

Any help would be greatly appreciated.