Try strawberrypunch.com with one device on a mobile network and your normal computer,
to enable online multiplayer functionality click or tap the planet button at the top left,
and you'll see they connect and can play together!!
it uses STUN and TURN server. It'S easy enough to get one running, get a 5$ a month droplet on digital ocean and follow a tutorial on how to install some open source STUN/TURN whatever server and you got it !