Internet shuts down after a few minutes – firewall suspected

Check if your ping is going through the VPN tunnel in VPN2 qube instead of going directly.
If your VPN in the VPN2 qube is dead and you have a firewall rule there that is blocking forward connections outside of VPN tunnel, then that could explain why ping works in the VPN2 qube itself and doesn’t work in the Firewall-INT qube.