If I understand this correctly:
1. You bought your daughter a mobile device, tried to log in using her Samsung account and it wouldn't recognise the device or the account due to the region????
2. You then said you had the same issue with a tablet.
2.1 Now you are saying that he tablet is fine when you log in, presumably using your own Samsung account then the tablet is working fine?
Have to tried logging into the mobile device using your Samsung account? If that works fine then the issue is with your daughter's Samsung Account.
Whichever of the above is correct you will still need to speak to Samsung to either resolve the Samsung account OR resolve the device.