How to play via LAN?

My son and I used to play LAN games all the time before Reforged. We just tried setting it up after the update, and it’s not working.

Either of us can create the LAN game. When the other of us opens up the LAN game section, we see the game, but with only one bar (red) for ping. When we click on the game to join, it tells us that the game no longer exists.

These computers are on the same wifi network, and each of them individually has a good connection to the internet, so I assume they should be able to establish a good connection to each other.

Any troubleshooting tips? Do I need to buy 2 copies of the game just to play LAN?