Thank you for the screenshots!
I don’t see anything wrong on first glance. However my Find Sessions Advanced doesn’t have a pin “search lobbies” and I’m also on 5.5.4.
I’m also have a slightly different .ini setup but honestly, I don’t remember why I did it this way 
One thing, however doesn’t give me rest. Can you please use a hardcoded value for public connection on your create session node? Don’t feed it from a variable. I have seen this several times where this is fed from a variable but it actually turns out to be 0. I’m not saying that’s the case but it’s worth a try.
Here’s how my ini looks like
[/Script/Engine.GameEngine]
!NetDriverDefinitions=ClearArray
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="/Script/SteamSockets.SteamSocketsNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")
[OnlineSubsystem]
DefaultPlatformService=Steam
[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=480
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="SteamSockets.SteamSocketsNetConnection"
[ConsoleVariables]
net.CurrentHandshakeVersion=2
net.MinHandshakeVersion=2
If this doesn’t work let me know, I will WIP up a fresh project and test it on two PC’s myself, maybe I can narrow it down.