USB 3G, and two wireless networks

Here's my use case - I have a wifi camera in my car, an android headunit and a Nexx WT3020 with a 4G USB modem.

I can't change the camera which has its own wifi so ideally I want the Nexx to connect to the camera (and route traffic from the other network to rtsp so I can use as a reversing camera) and create a new wireless network that the headunit and phones/etc. connect to and then use the USB modem for internet access.

I can either connect to the camera and create a new network or create a network and use the USB but not combined. What am I missing, are there any recipes similar to this I could follow?