I use two SIM cards in my FP2, the same as before in the FP1 for many years. The primary one in slot 1, the second card only if I would have signal problems or similar (but both using the O2 network).
Recently I replaced the second card with a new one, but again O2 (https://forum.fairphone.com/t/o2-testkarte-fp4-5g/ (my first with a flat rate for 30 days ;-))).
But as so often the signal performance of this “second card in the second slot” is very, very poor, with luck “Edge”…
Now I swapped the cards - and “suddenly” I have LTE speed.
Long story short: are there (such) differences between the two slots or does my FP2 simply have a defective SIM2 slot?