I was able to get it to work it was actually really easy once I figured it out.
1 activate online subsystem steam plug-in.
2 paste this into your DefaultEngine.ini but replace the appId with your app Id and the names of the achievements with your achievements names. (make sure you DO NOT have any " quotation marks before or after the names!)
[/Script/Engine.GameEngine]
+NetDriverDefinitions=(DefName="GameNetDriver",DriverClassName="OnlineSubsystemSteam.SteamNetDriver",DriverClassNameFallback="OnlineSubsystemUtils.IpNetDriver")
[OnlineSubsystem]
DefaultPlatformService=Steam
[OnlineSubsystemSteam]
bEnabled=true
SteamDevAppId=2273220
Achievement_0_Id=Died_ACH
Achievement_1_Id=PortalUnlocked
Achievement_2_Id=CarlaHelped
Achievement_3_Id=MileHigh
Achievement_4_Id=GideonHelped
Achievement_5_Id=Indomitable
Achievement_6_Id=Unstoppable
Achievement_7_Id=Impregnable
Achievement_8_Id=Completionist
Achievement_9_Id=DefeatFrank
Achievement_10_Id=DefeatFrankOnInsane
Achievement_11_Id=AncientKill
Achievement_12_Id=1kKill
Achievement_13_Id=YinHelped
Achievement_14_Id=PhaseHelped
Achievement_15_Id=Unassailable
Achievement_16_Id=Insurmountable
Achievement_17_Id=Psychotic
Achievement_18_Id=Test_ACH
; If using Sessions
; bInitServerOnClient=true
[/Script/OnlineSubsystemSteam.SteamNetDriver]
NetConnectionClassName="OnlineSubsystemSteam.SteamNetConnection"
3 You should now have a Steam overlay when playing in stand alone mode if not download and activate the free steam advanced sessions plugin.
4 use the “cache achievements” BP node plugged into get player controller and “write Achievement progress” node with the name your ID= like Died_ACH to make your achievements work.
5 Jump for joy when Steam shows your achievement popup
Hope that helps if you have any questions don’t hesitate to ask!