Burp – How to capture local traffic from Android device (i.e traffic that doesn’t use the internet)

I’m trying to intercept traffic between an Andriod App and a Router interface. I’m using a rooted device.

They communicate on port 6699 which I believe is typical for nginx.

I’ve tried setting up Burp to capture the traffic but all it sees is the data the App sends/receives remotely (API calls to the cloud) and nothing it sends/receives locally (API calls to nginx on the router).

It seems like the Burp listener port is only capturing traffic from ports 80 and 443 on the Android device.

How can I set up burp to capture traffic from port 6699?