@tenderfoot 

How do you find out the MAC address? on the client device or on the admin portal of the router? and if you connect directly to the main router, will it work?

For some TP-Link range extenders, when you are connected to the extended network, the MAC address will be changed, not very sure if the TL-WR840N in extender mode behaves the same or not, you could connect a device to the extender, then check DHCP Client list on the main router to see if the device MAC address is the same or not.