I haven't figured out a solution to this issue, but I do have a workaround.
For whatever reason, it's impossible to join a game being hosted on the same network, dedicated server or not. This is true even for a dedicated server being hosted on the same computer.
You can still host a dedicated server, port forward, and have it be accessible to friends on different networks at different locations. In order to be able to play with those friends--to connect to your own server--the only workaround I've found is to host a dedicated server on a separate computer and port forward like normal; then on the computer running my game, I use a VPN to trick the game into thinking I'm on another network.
I use Nord (nordvpn.com) to do this, it's very simple--just install the windows application, run it, and click on any country. You should be able to connect to your server as normal. Unfortunately Nord isn't free; this might work with other VPN's but I haven't tested any so I'm not sure. For LAN parties, everyone can use the same Nord account to connect to a dedicated server being run on a separate computer.