@cyrusShould be correct - grabbed from the official site and verified the sha256sum. Only reason why I thought about the SD-card is because I suddenly remembered that the guide said to remove SIM-cards and me thinking "well that's stupid, why would I ever need to do that?" before flashing :P
Also tried enabling roaming on the phone, setting the mobile network manually, resetting APNs, rebooting etc. to see if any of that helped (which it did not).