The first question would be why does the remote Vmix need to be the listener?..
Usually these sort of architectures have a central studio or stream accepter, and the remote LTE locations connect into that central studio for processing.
It seems that you are working on a remote studio, where you want to source some other video for processing on a remote workstation.
So, you can either have a central hub, that both the remote video source and the Vmix console connect to… or you will have to create a system that allows your remote network to have a fixed presence on the internet.
You are using SpeedFusion Cloud right now to test… that is an outbound only service…
For an inbound architecture you need to run your own FusionHub.
Martin has written some guides on how to run that on Vultr ($5/month) and others have AWS and Azure guides.
You can search for various FusionHub configurations, and quite complex networking is possible.
This then gives you a fixed IP on the internet, that you can forward to your port 5001 to the IP address of the VMix console. The port forwarding has to happen on the FusionHub. where you have a fixed IP… which is why you can’t do that on the Duo… the LTE and speed fusion Cloud parts of the DUO do not have fixed IP addresses.