RBK852 with EAX20 / AX1800 wifi extender

Someone gave me their un-used, in sealed box AX-1800 Extender (EAX20-100)

In my home, I use one RBR850 and two RBS850. 
I could use this AX-1800 for the far end of the garden. but is this compatible? if it's slower than the orbi's - will the entire setup default to the slowest item (i.e. the AX-1800).
Also, will it show up in the Network Map of the Orbi App? I see on the box that it needs the nighthawk app.

Thanks!