Is anyone using Orbi RBK 852 with comcast business gateway

Your kind of caught with a problem that'll either require an investment (another switch) or to run the netgear in access point mode. 

Right now you're in a double nat. 

A double nat happens when you cascade more than 1 router.
https://kb.netgear.com/30186/What-is-Double-NAT
Fixes? There’s 3 options but only use 1.
1. Put the modem/gateway in modem only/pass through mode. This disables its routing functions. But then you can't use the ethernet ports other than the 1 to connect the netgear to. 
2. Leave the modem/gateway in router mode but put the Netgears IP address into its DMZ. This forwards the data to the netgear. You can either leave the modem/gateway’s wireless enabled or disabled. Just make sure its using a different wifi channel to prevent interference. Problem with this route is the 2 devices are still in seperate subnets and devices on the seperate ones might not be able to see each other. 
3. Put the Netgear into access point mode. This disables its routing functions. Wifi/ethernet still works but it does disable some features.
https://kb.netgear.com/26765/Which-features-are-disabled-on-my-NETGEAR-router-when-it-is-set-to-AP-Mode

Problem with this route is the features that get disabled on the netgear. If you use this features, then its a challenge. 

4. Put the comcast device in modem only, run the netgear in router mode, and add another switch. 

Kind of up to you which route you use.