Green1

We don't live and breath a buggy mess of a mod whenever it changes. Most of our customers use vanilla Valheim with no issues. If it changes and it breaks, you are aware that adding a BETA mod to your server will cause issues and it is up to you to fix them or you can create a ticket and we can point you in the right direction.
Our Modify Valheim+ Config tool is not magically updated with new config options each time an update is released. We add them manually. It will take days. If you can't wait days, get an FTP program and modify the BepInEx\config\valheim_plus.cfg yourself by copying the latest changes from valheim_plus.cfg (use Winmerge or similar) on Github and every Valheim+ update will be a sinch.

Is there any way to prevent users from using trainers (cheats)? I have people joining my server with cheats on even though they have no access to admin rights.

Trainers run locally. Best bet would be to ban their SteamID from your community/server.

Since this isn't working what's the next step? 

This is not working either. 

The direct connect method doesn't solve the "Wrong password" issue. 

In the "Modify Server Config" You go almost all the way down. And you will find "Player Levels Configuration" Press SHOW
Here you can enter all the levels you want and the desired XP for that particular level.
Then go to "Override the Max Experience points for players" and enter the Highest XP (For example if your max is lv 200, 300k xp) fill in : 300001
 
That worked for me.
 
For the XP, just enter 30x and you will level 30 times faster.

XP multiplier and Overiding the Max Experience points for players?
 
What should these numbers be, if we want a 30x server for xp, and 300 levels.