Solution
To add a TPC-RIO to your project, it must be connected to an MTPC via the TPC-POE port on the on the RIO. The TPC-RIO and controller share an IP address.
This a point-to-point connection meaning it must be a direct connection from TPC-POE to the MTPC, this connection cannot pass through a switch.
Once the MTPC is connected, the controller should show up in the network tab of Mosaic Designer.
Select the MTPC , navigate to the interfaces tab on right hand side of the workspace.
Choose "Configure TPC-RIO"

Save and upload your project.
The device type should now change to MTPC + TPC-RIO in the network tab.