For those people looking for a solution to this (and who happened to skip the first google result and got this instead like I did), check out the guide here Network testing across multiple computers - Blueprint - Epic Developer Community Forums
Basically:
- Package game using Development config
- Run the packaged game on the host computer, open console (~ key) and type ‘open [mapname]?listen’
- Run the packaged game on the client computers, open the console and type ‘open [ip address of host computer]’