You have multiple possibilities, the easiest being a vpn.
This one is easy to set up: https://vpncloud.ddswd.de/

You can follow this tutorial, but keep in mind that it uses an old version of vpncloud: https://matteocontrini.medium.com/how-to-setup-a-peer-to-peer-fully-meshed-vpn-between-servers-249a9071e714

Be sure to use tap mode, since ros2 discovery works with udp broadcast. Once you can ping the machines, you should be able to find each others nodes and topics.

Another way is to use a fastdds discovery server, according to the following tutorial: https://docs.ros.org/en/foxy/Tutorials/Discovery-Server/Discovery-Server.html

Be aware that using command line tools like ros2 topic list does not yet work out of the box, see https://answers.ros.org/question/373070/ros2-topic-empty-with-fastdds-discovery-server/