Security Updates not working

[Scotophor]

I have an S23 Ultra, U.S. model (SM-S918UZKFXAA) bought used and unlocked. I'm in California on Tello MVNO (under T-Mobile). My phone's settings say "Security patch level: July 1, 2024" and that it's up-to-date, which I'm sure is not true. I can't get the security patch to update.

I've read other posts here saying to power off the phone, remove the SIM card, restart, and try to update without the SIM. That hasn't worked. I also read to try another SIM but I don't have another SIM available.

Is there any other method to get a reluctant phone to update its security patch?

[meself]

Smartwatch i think the app is called seen mentioned to help

[3Fees]

Samsung Smart Switch. Connect your phone through usb cable to pc with Samsung Smart Switch installed on pc and then check for updates.

Cheers
3Fees

[Scotophor]

Check for updates through the phone's Settings like normal?

[3Fees]

Look here :

https://www.samsung.com/ae/support/mobile-devices/how-to-update-software-using-smart-switch/

Cheers
3Fees

[Scotophor]

Oh, it's a feature inside the Smart Switch app. Thank you!

[Scotophor]

Hmm, I finally got to try the Smart Switch app method (from a Windows laptop), and it doesn't work either. It doesn't give me any option to update the phone.

Is there anything left to try myself, or should I take it to a service center?

[3Fees]

You have to click on update and if there is one from your carrier, a download will happen and install is next.

Cheers
3Fees

[Scotophor]

This phone is still supposed to get monthly security patch updates. In a few days, the June 2024 patch that it's currently on will be 9 months old. Every method I've tried to make it update tells me that it is up to date, when it clearly isn't.

The most recent method I tried was to boot into Safe Mode and update through Settings (under both "Security and privacy", and "Software"). It did exactly as before, claim that it's already up to date, and not offer any updates.

[Scotophor]

One thing that I noticed that might have something to do with this, is a model number mismatch between what I found in the phone (I don't remember exactly where) and what Samsung has it registered as according to their website.

The phone says it's:
SM-S918UZKFXAA

Samsung says it's:
SM-S918UZKFATT

It was originally a carrier-locked AT&T phone but is now unlocked. Is this the reason for failure to update? Will it forever only update when on the AT&T network?