I'm sure someone has done this - Sophos XG gateway PLUS EdgeRouter Lite

Here's the scenario.

I have Sophos setup to handle ALL routing and firewall activities for the business network but I also need to maintain a separate network where I can build and test various scenarios and devices. This includes requiring an EdgeRouter Lite between Sophos and the Lab network devices. To the EdgeRouter Lite, the traffic coming from Sophos should look like it's coming from the internet. In other words, I want to be able to setup the EdgeRouter Lite so I can lift-and-shift for setup at remote locations.

I realize this places me in a double-NAT scenario but what's the right way of doing this? Essentially, I need the EdgeRouter Lite to act like a router and route traffic to the Lab devices which will eventually be installed at client sites where I may not be on location. They will plug devices according to my directions but to document everything properly, I will be setting everything up in the lab ahead of time.

Sophos needs to continue to handle routing, DHCP and firewall activities for the business network but EdgeRouter Lite must handle routing for the lab devices.

I'm guessing someone has already set something like this up. Just looking to see what the best setup is to do this. I believe it's called mixed mode but not sure.