Hi Syrus_BD. This sounds like an interesting project for sure.

I guess the first thing I'd suspect would be firewall / router related issues. Perhaps the necessary ports are being blocked or not being forwarded?

Depending on how your VPN is set up, port blocking / or  non-forwarding of ports on inbound / outbound traffic can still occur... even with the tunnelling. What kind of VPN are you using?

Further to that, perhaps bandwidth?

Maybe the queue manager needs a minimum bandwidth for its network communication to work?