It could be a port forwarding issue or it could also be because of the windows firewall (assuming you are running windows).
Make sure TCP ports 3000 and 3001 and forwarded from your router. Then, either disable windows firewall, or add ingoing+outgoing rules for ports 3000, and 3001. (Replace 3000, and 3001 with the different ports if you changed them in the network config at all)
Even just playing (not hosting a server) uses a lot of data when using the phone as a hotspot and most phone companies are cracking down on large amounts of data use. For example Verizon is just dropping people who use over 500gb a month.
It took a lot of data, was painfully slow, and because I had a working router at home, better for me to remote back to my computer and log in. Only using the hotspot when I had to use my chromebook/linux to get my character to turn.
Jhadred is correct. However, on my dedicated server single player playthrough the skills bugged. The carpentry never showed up after learning, and so I couldn't build roads. Took me a while to figure out what I was missing, though. :p
Unless you set your server to Private = True, then anyone can join since it's just listed in the server list.
I expect it's just another player who just happened to join your server, either on purpose or by accident.
If you do set Private to True, make sure you add the players you want to your whitelist. Spelling including upper and case matters. When I added "jhadred" to my whitelist, I couldn't join until I figured out my name comes across as "Jhadred".