The Virtual Server is a basic and easy function of the router. If you can sure you have set it up successfully, how do you know it does not work? I means there may be some other factors that cause...

Why not contact the supporter directly for help?

If you use L2TP/IPsec VPN in TP-LINK router, you need not to specify any encryption methods for IPsec. The system will use the default one to encrypt the L2TP tunnel.

That is odd. You can try to reset or reboot the devices and then bind it again. If the problem still exists, you can contact the supporter for help.

Do you mean the Link Backup function? In your situation, you can set the Link Backup as Failover mode.

I think it can not do this.