Thanks for chiming in! I'm very interested in getting all the various IoT devices on their own network (VLAN?) as I've wanted to understand better their impact on my network and have some concerns about their security vulnerabilities. Hopefully that is something I can figure out how to configure.I think you will be happier using UniFi over consumer gear for all the reasons stated above. You can have higher densities for more WiFi devices using separate APs and more of them. You should inherit roaming by using all UniFi wireless.
I setup 3 Cisco wireless APs for my daughter's house using their AT&T router modem. I was going to buy a router and replace the AT&T router but it worked well and I never did. So you could use your ISP's router modem with a UniFi wireless network. The ISP keeps the latest firmware on their router modem. This is providing you can turn off WiFi on the router/modem.
So the question is can you turn off the ASUS wireless? Is it still supported firmware? Make sure you have the latest. It may be a time to switch over to a UniFi router more family friendly. Using an UniFi router you will be able to add guest networks and IoT networks and things the ASUS does not support. This is more advanced networking but doable.
I can turn off the WiFi on the Asus, but I'd need to have a replacement wireless option to cover this part of the house. Looking at the FlexHD and NanoHD as possibilities. I am running the latest firmware and I believe it is still supported.