Mobile data simcard always resetting

[1man2simcards]

Hello Community

I got a new Samsung A51 and so far I am very happy with the device. However, I got a major issue, that I couldn't solve yet. I have two SIM cards. Initially I set up SIM card 1 for calls and texts, and SIM card 2 for mobile data. This was working without a problem. Now I changed country, and I would like to use SIM card 1 for mobile data as well. This was working initially, however at a certain point, without telling me, mobile data was reset to SIM card 2 again. I didn't realize it and was continuing to use the phone, which will lead to a rather hefty bill at the end of the month. I can change to SIM card 1 again, but it alway seems to revert to SiM card 2 at a certain point. I couldn't make out the pattern when it happens, so I always need to check.

Did anyone experience a similar issue? Maybe an idea what I could be doing wrong? Any help would be much appreciated. Thanks!