Get a free SIM card from any of the main UK networks (or pick one up in a supermarket for less than £1). For ease, remove existing SIM. It will only be for a few minutes.
Pop it on then Turn WiFi off.
Restart your device.
Let it connect to 4g / 5g for a while (maybe 20 minutes) then turn WiFi back on and try to update.
If updates start to come through, hopefully you'll have fixed the issue at very low cost
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.