If they were able to guess your pin, you should probably switch to a longer pin or a password. It seems insane at first to type a long password, but if you pick two long scientific words, its secure with only letters.

What I said, a smartphone can never be a secure device and this is why it is crazy to have sensitive data stored in these gadgets. It is certainly unlikely that this person gained access by trying the pins, because the cell phone would be blocked after the third failed attempt, but even so, a computer expert needs seconds to access, bypassing this little protection that cell phones have. You can be happy that your Secure Banking app is better protected, probably with 2FA, coordinate card, ID card or similar. In any case, this mobile phone is already compromised, which makes it necessary to change at least the credentials, better even your phone number (ask your ISP). Also be attentive, since this person may have been honest, but he returned it may also be because he was not interested in the cell phone, but in the address where you live.