I also started having trouble with 4G (tethered network with phone was working for a couple of days on my laptop without any problems).
Today I got a random DC and after that I could barely login only to find myself with 15k ms lag and more disconnects. I’m assuming blizzard have done some filtering on their side to somehow restrict mobile networks.
In any case, I tried a VPN on my laptop and the problem seems to be fixed. Any official Blizzard response would be appreciated.