UE4.19 Blueprints/C++: Steam cannot join session

I have a project running UE4.19 with Blueprints (primarily) and C++ (for Discord RPC). In my blueprints, I try to join a Steam session. When I Play In Editor (Standalone) or package it and run it twice, I can host a Steam session from one instance and join it from the other. However, when I package it and play it from two computers, I can find the other session, but joining it doesn’t work (nothing happens). Why might this be?

Join Widget: For UE4 Answerhub (Join Widget) posted by anonymous | blueprintUE | PasteBin For Unreal Engine 4

Hosting Widget: For UE4 Answerhub (Host Widget) posted by anonymous | blueprintUE | PasteBin For Unreal Engine 4

Spawning Join Widget: For UE4 Answerhub (Search Widget) posted by anonymous | blueprintUE | PasteBin For Unreal Engine 4