[copperx]

I'm ashamed by this Networking 101 question, but what prevents you from connecting the UAP-AC-Lite directly to the ISP's device? (Assuming you don't want a physical ethernet connection at all). Is it for DHCP and assigning IPs to the clients?

[Macha]

Nothing. I do this. I think the AC-Lite even has it's own DHCP, but I'm using the ISP router for that personally.

Usually the ISP router just sucks at wifi, but I have seen ISP routers which have only 100mbit/s uplink ports when the internet connection is higher. In that case you'd want a custom router also. Or if they ship some router with some features you dislike that you can't disable (like public hotspots, unpatchable insecure config interfaces, etc.)