You may also adjust the wireless settings on your router. Log into the router, find the wireless settings menu, there should be a option called channel width or HT mode, select 20MHz for better...

Try WDS repeater mode.

http://www.tp-link.com/resources/software/TL-WA850RE(EU_V1_140826.zip Try to upgrade the latest firmware and reconfigure it again. Still use mode 2.

Hi @JamesIT It sounds like you are using Client mode. Both AP mode and Client mode work as a pure switch which will pass through the DHCP traffic from the front network (Netgear R7000) to the client...

Can the clients which are connecting the extender access Internet normally? Only ping fails?

Upgrade the latest firmware from the official website. Place the range extender closer to the main router, avoid obstructions in the way.