Triband and Aimesh are two different things and have nothing to do with each other. The router only has to support Aimesh. Since the ASUS ROG Rapture GT-AC5300 is a VPN router, it has the corresponding processor line, which is also noticeable in the transmission speed.

* Yes, I meant.

No.
You can find the supported routers at https://www.asus.com/AiMesh (scroll down).