How to handle player connection using SteamSubsystem

I am making a coop game peer to peer through steam. I have a lobby level and a gameplay level using Server Travel.

Here how it going:
Host enter level first then the game start immediately.

What I want:
Wait for all player done travel then start the game

Problem I have:
Client joins late and miss some events when the game start

What I have done:
I check player in previous level and check in current level if everyone joined

New problem rise:
What If player disconnect before done travel