I'm really struggling to reset an old/spare phone (a OnePlus Nord) my son needs to use for a couple of weeks; it was my wife's and clearly is still set up with an account (ie we didn't re-set it prior to it going into the Drawer Where All Unused Phones Go To Die). The problem is she and I can't recall what the passcode was; and, while I've tried a hard reset, it still comes up with needing to enter a PIN (and this PIN isn't the same the phone gave me to enter at the point of resetting it).

And clearly there's still an account linked as, while I can escape out of the PIN screen, it asks me to enter the details of a linked Google account. I've tried a range of our own Google accounts but none appear to be 'the one', as even after entering the username and password and confirming on a linked phone it just returns to the same login screen (not even confirming it's not a linked account). This has left us with the assumption we must have lent the phone to someone else, but we don't know who!

Can anyone provide ideas on a solution to this? Ideally I'm just looking for a way of fully re-setting the phone so that it has no account attached and can be set up as new.

TIA.