Thank you for clarifying.
I've contacted the carrier and, as expected, they doesn't see any issues with the SIM or service. I wasn't able to get clear answer if they support specificaly Cat-M1 or not. Since LTE is working in a mobile phone, for them it's sadly a closed thing.
I hoped there could be a way to check what the modem is actualy seeing, like bands, signal strenght in dB etc. I imagine, from all these parameters, it would be simple to determine what network category is used.