From a programming perspective there is no difference between LAN play and Internet play. In both cases you just open a port and start listening.
The problem with Internet play is that, despite it being 2026, it is still an absolute bitch to get an Internet connection between two people running flawlessly. On top of opening a port, you need to make sure the port is forwarded correctly by your player’s router. And then, if the player is behind an ISP with a CGNAT, then that must be configured somehow as well. There’s tricks like STUN, but none of them are 100% effective and most of the time you have to fallback to a relay server you, the developer will need to host 24/7.
In theory this is something UPnP IGD, NAT-PMP and their successor, the Port Control Protocol, should be able to handle, but in a twist shocking to no one, few routers support these, and the most widespread one, UPnP, is disabled by people who have been scared into doing so without understanding why.
So yeah, in practice your players have to forward their ports manually, and that means having some basic networking knowledge, and that means just overall not being stupid, which is not going to happen.
All of that you will avoid, if you claim to support only LAN play. That being said, your players can get a VPN a la Hamachi, make a virtual local network, and play “LAN” through the Internet anyway.
EDIT: Oh, and IPv6 could not come soon enough. If I am made president