What's your phone OS? I use Lineage OS 14.1 unrooted (equivalent to android 7.1), my friend using iPhone 5, both working using WiFi.
You can try using USB tether, maybe it working.
Actually it kinda strange, when I using USB tether I can browse internet, play other game, or any other network activities, just can't login to this game. When I tried to sniff the network using Wireshark, the request packet is equal when I login with WIFI, so idk what the problem is. Maybe server restriction on some source packet for bot protection.