Your phone came out five years ago. Samsung has discontinued support for it. There are no more updates after May 2024. Those types of phones usually get four years of updates. S-series phones get seven years of updates (newer ones, anyway).
Your options are to carry on with no more system updates or get a new phone.
I should also add that Samsung does not sell M-series phones in the UK. Having a phone from a different country with a UK SIM card prevents updates, though that is not your issue here.
M31 is a very good phone, sadly is no longer on the Security Updates list on the official Samsung Site. It will function as normal, but it's more prone to Security failure. Is up to you if you want a new phone or you continue to use your phone longer.
