Hi all, I'm a long time Samsung user (started with an S2, then S4, S7 and now S10e) though this is my first post here. 

I'm in the UK. A couple of years ago, I bought a Samsung Galaxy S10e from an ebay seller. It all worked fine and the whole family ended up getting S10e's to replace their aging S7s and S4s from the same seller. They've been working fine, except only one of them receives Android updates. 

It turns out these were AT&T phones, yet they are unlocked and we're all using UK carrier SIMs fine.

One is up to Android 11 and regularly installs updates still. The other 3 phones are still stuck on Android 9.

I haven't been able to contact the original seller.

I noticed the "Service Provider SW ver." is different on the ones that don't receive updates. It says

SAOMC_SM-G970U_OYN_ATT_PP_0036 R38M20GMFRZ ATT/ATT/ATT whereas the one that receives updates says SAOMC_SM-G970U1_OYM_XAA_RR_0007 XAA/XAA/ATT

Do I need to update the Service Provider string to be able to receive updates?

If so, how do I do that? Or do I need to do something else?