SSL-certificate error on mobile data

Hey!

So I've got a family member who is not able to get help from these types of forums so I'll do it for her.

When she tries to use any form of mobile-data (not WiFi) she gets a SSL-certificate error on whatever she tries to do. It can be access a banking app or browsing with Safari (the error message is different based on the app used but it's always something along the lines of "Could not establish safe SSL-connection)

Here is a check list of information and solution attempts:

1. The phone is updated to the latest version of iOS.
2. The entire phone has been wiped and reset to factory default.
3. Tried another iPhone (iPhone 13) (non-carrier locked) still same SLL-error
4. She has ordered an entirly new physical SIM-card and changed to that.
5. New/Old SIM-card does NOT work in ANY phone, not just this iPhone.
6. Carrier claims nothing is wrong on their end.
7. Her phone/SIM-card setup has been working flawless for 1 year and started acting weird 1-2 weeks ago.

Note: This is not an SSL-error associated with e-mail or any specific service. Her e-mail works on the phone when on WiFi.

So I guess the questions then are:

1. Can something be done to fix this kind of SSL-error on the user side of things i.e. do something in the phone settings?
2. Is this a carrier problem after all and up to them to fix? And out of curiousity, what may have happened? Lost certificate?

I'm sort of techy but this seems like an error that could be difficult to fix, on either side...

Thanks for any help!