Can the Nexus 6P use GSM data if the connected Wifi has no route to the internet?

I have a Nexus 6P and use it to connect to a Phantom 3 4K drone using WiFi. Obviously, because it's a private WiFi network with no external connectivity my phone can not contact the internet via WiFi.

This wouldn't be a problem but the app that I need to use, DroneDeploy, requires a connection to the drone via the DJI Go app (which needs to use the WiFi to connect) and the internet to pull down waypoint settings and to use mapping images.

I have a 4G connection however when my phone connects to the WiFi it doesn't seem to try and fall back on the GSM for external connectivity.

Is there a way I can get my phone to use the 4G connection for the internet connection while maintaining the private WiFi link to the drone?

Thanks in advance!