@Bomber_21 

You mean WEB editor is connected to MR6400, right?

If that is true, please check the Internet IPv4 address of the MR6400 first to see if it is a public IP address or not, you could find it on Advanced > Status > Internet part.

If it is a public IP address, you could use NAT Forwarding > Virtual Server to open port for the WEB editor to allow external access to the server directly:

How to share local resources on the Internet by Virtual Servers with TP-Link Wireless 4G LTE Router

Or you could also enable a VPN Server such as OpenVPN Sever on the MR6400, then connect your mobile phone to the VPN Server first and then access the WEB editor as if you are in the same network.

How to set up an OpenVPN connection with TP-Link Wireless 4G LTE Router

However, if it is a private message, it means MR6400 is behind your ISP CGNAT, which will block the external access, thus NAT forwarding or VPN Server will not work, you will need to contact your ISP to see if they could assign you a public IP address.