I hesitate to disagree, but configuring the secondary WiFi unit as a 'router' would create a double NAT configuration, no? Is this intentional, to prevent the computers on the second WiFi from easily 'seeing' the primary local network?

Personally I prefer to utilize the 'guest network' facility in a modern router to create two WiFi networks with separate SSID but central control and monitoring of everything from the primary router. In my current case these happen to be Apple gear, AirPort Extreme and Airport Express. Certainly easy to manage , and for me they have tended to be very stable over time. Pretty good signal quality and range, the stability and consistency has been the attractive aspect.

I long ago tired of the bleeding edge WiFi gear that needs multiple updates before it becomes trustworthy, if it ever gets there. And the really cheap stuff is all over the map, can consume a lot time and energy sifting through all the crap looking for something that won't become trouble later on.