What's the hardware version of your VR900? On the V1, it has WDS function which can change the router to extender, but this function may not work well sometimes. For V2 and V3, they don't support WDS. So they cannot work as wifi extender. For the configuration of WDS, you can find it on the UG, page 105: http://static.tp-link.com/res/down/doc/Archer_VR900(EU)_V1_UG.pdf

Dear Anton, thank you for your response. I have V1 and have managed to set up WDS. It's when I assign a new IP address that things go belly up. I know it cannot be the same as the root router ie 192.168.1.1, but when I change the last number to 110 for example, I loose access to the router and it cannot be found.

If you change the IP on the VR900, you will need to use the new IP to log in to VR900. After you change the IP, and disable the DHCP server, you may not be able to log in the VR900 if the connection between VR900 and VR2800 is not established successfully.

Thank you again Anton. I have done all of the above. I will keep trying.