How do I connect a 2.7gh camera to the AC3000 tri-band wi-fi model R7900P router?

Insufficient information for anyone to answer your question.

Start with the brand name and model number of the camera.

What camera is it? 

What modem/gateway is the router connected to? 

What actual issue are you running into? Because usually you just use the app install process that the camera manufacturer recommends. If that doesn't work details of where its failing helps.

also, did you contact the manufacturer of the camera for help with installing their product?