In DefaultEngine.ini you need to set a few things.
[OnlineSubsystem]
DefaultPlatformService=Null <-Replace Null with Steam if thats what you want to use
PollingIntervalInMs=20
[/Script/OnlineSubsystemUtils.IpNetDriver]
InitialConnectTimeout=120.0
[/Script/Engine.GameEngine] <- Here I commented out the Null subsystem line, use whichever suits you
!NetDriverDefinitions=ClearArray
;+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="OnlineSubsystemUtils.IpNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="OnlineSubsystemSteam.SteamNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")
[OnlineSubsystemSteam] <- Only need this category if using Steam
bEnabled=true
SteamDevAppId=480
GameServerQueryPort=27015
bRelaunchInSteam=false
GameVersion=1.0.0.0
bVACEnabled=1
bAllowP2PPacketRelay=true
P2PConnectionTimeout=90
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="OnlineSubsystemSteam.SteamNetConnection"