I don't know where in Europe your tablet came from, but European frequencies are fairly uniform so I'll assume that a UK model is representative. In which case according to Samsung it should support LTE bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 20, 25, 26, 28 32 and 66, while according to an ATT forum post they use bands 2, 4, 5, 12, 14, 17, 29 and 30, with 12 and 17 being the main bands. In which case you should have LTE compatibility. Samsung also claim that it should support 5G bands n5 and n77, which are used by ATT (plus others that are not).
So if the 3 sources I used here are all accurate (and that's not certain: manufacturers have themselves been known to post inaccurate information by copy-pasting between their regional sites!) you should be able to use 4G or 5G, provided you have coverage with the right bands.
You say that the APN settings change when you insert the SIM: do those settings look correct for ATT?