Once LTE works, it generally stays working. e.g. most often issues some firmware mis-match or some bad interaction with some carrier. Just test any future upgrades on a less “delicate” hAPliteLTE6 before upgrading more critical ones – as upgrades, once in while, still mess something up in V7.
Still unsure why @BartozP is having trouble, be curious to hear what mikrotik comes back with.