It’s probably an incompatibility with the wireless providers’ network (preferring IPv6) with Blizzard’s choice to implement their mobile app services with IPv4 only.
I assume Blizzard will add IPv6 compatibility at some point, but if you don’t want to wait around the linked post has a cross-post to the Hearthstone forum with a detailed workaround (reconfiguring your mobile device to enable IPv4 compatibility and connect to Blizz mobile services as they are now).