Over on the JTS website, in the Q & A section, they posted this.
We want to play over the internet with a direct connection, but are having trouble doing so. What specifics can you share?
There are a variety of challenges when trying to make a direct connection accross the web, the biggest being firewalls (security software). We can't give you specifics on each particular firewall, as there are tons of them, however we can point out that you need to have the following ports open for UDP/TCP traffic:
47624 to 47624
2300 to 2400
6073 to 6073
If you do this you should be able to connect to your opponents computer and conduct a real-time game. People have done it successfully in 2020.