Add to AX1800 Mesh Network

[RickyO]

I'm using an ASUS ZenWiFi AX 1800 Mesh wireless network on a home network (with Internet through wired connection to modem). I'm not getting the coverage in my garage that I need to connect Rachio irrigation controller. The second node of the AX1800 is against the wall between the house and garage (attached garage).
I have an ASUS RT-AC5300 I used to use, and would like to know if I can attach it to the AX1800 network; and if so, the proper steps (step-by-step).
Any help appreciated, Thanks!