Adding RBR50 to Existing Network

Try this, Disconnect the RBR from the main host router as it doesn't need to be connected at this point. 
Have your wired PC connected to the back of the RBR. Use a web browser, IE11, Edge or Firefox and navigate to 192.168.1.1. If you still can't access the RBRs web page here, try factory resetting the RBR by pressing the reset button in back for 15 seconds then release. Try 192.168.1.1. You can manually setup the RBR when you see the setup wizard begin. After you get to the RBRs main web page after the setup wizard complete, go in and set AP mode on the RBR. Advanced tab/Advanced/Router-AP mode.

I highly recommend setting a static IP address on the RBR so you can access the RBRs web page while in AP mode at any time. I would use 192.168.#.50. This IP address needs to be outside of the host router default IP address pool. I use 192.168.#.100 to .200. You may have a different IP address string and pool size for the business needs so adjust accordingly.

Once you have the RBR configured for AP mode and you apply the setting. The RBR should reboot. While it reboots, connect the LAN cable from the host router to the RBRs yellow WAN port. You should have internet access thru the RBR in AP now.