the MR6400 is a 4g lte router. Most cellular providers do not provide a public routable v4/v6 address. The IPs are typically NATed. This makes is difficult to connect to the local network using the VPN options OpenVPN and pptp.
Thus, please provide an option to _initiate_ vpn connections to an external host. I'd prefer an ipsec v2 connection that the lte router keeps alive. This would allow to connect to the lan using the tunnel even with an private IP of the cellular connection.
I'd assume that some keep-alive is required so that the NAT does not close the connection.