R7000 as a range extender

Hey all,

Trying to do something simple but not quite sure how to do it.
I have another router as my main router, with a 5GHZ wifi.
I want to take the R7000 (with tomato on it) and turn it into an extender where it will connect to the wifi, and supply a connection to the ETH ports.
meaning my workstation will be connected via cable to the router, and connect to the internet riding on the wifi to the main router.

Can anyone point me how to do that?

thanks!