Welp I've finally managed to solve the demo problem with help from Steam Support.
For those not in the know, I recently added a free demo to the game that would allow players to play the game with restricted access (no cosmetics, no level editor, etc.) but they could still host and join lobbies.
I hoped this would help alleviate the player base problem we've been having recently, in that there weren't enough paid players playing, so there weren't enough lobbies to join, so paid players were refunding, and the cycle continues.
And theoretically, this change would solve that problem, as the game had a lot more lobbies and players when it was free. It would also allow me to keep working on the game as there would still be some funding from those that would purchase the full game to unlock cosmetics and such.
The only issue: Steam doesn't allow demo players to connect with paid players and vice versa.
(They should really make that text a little bit bigger in the documentation)
So, after a week or two talking with Steam Support, I came up with the solution to remove the demo, and instead make the base game free, where you can optionally purchase a DLC to unlock full access to the game. The only problem that I needed help with is giving the DLC to all the players that had already bought the game, which is why it's taken so long to get this fix out.
So hopefully now you should be able to play with demo and paid players alike, and if not, I'm going to go cry in the corner.
Anyways, let your friends know and hope you have fun playing Tank Team!
Regards,
A Terrible Dev