We would like to use a TL-MR6400 as an LTE backup router in case our DSL line fails.

Behind the LTE_Router is a Sophos firewall.

This establishes VPN connections to other locations.

I cannot get the VPN connection to work via the LTE router.

I activated "DMZ" for the internal IP address of our Sophos firewall in the LTE router, so that it is now exposed host.

Nevertheless, the connection does not want to be established.

The VPN works problem-free over the DSL line. For the test, of course, I adapted the peer address on the other side to the public IP address of the LTE router and selected the interface on the Sophos firewall as the outgoing interface where the LTE router is connected.

Have I overlooked something in the TP-Link router or is it basically not possible with the device?

Thanks for any help.