It appears to be something to do with using PAYG SIMs for mobile broadband. Back when I lived ten minutes from one of the largest cities in the country I used 4G, but didn't run into this or their CGNAT crap because I tunneled out to a sane ISP.

Given that you can buy a SIM that'll give you a couple of hundred GB of data for under a tenner, it seems reasonable that they'd block stuff you didn't want young children getting access to (easily).