At some point (I think it was when I needed to track down what device on my network was consuming multi-gigabytes of data daily), I moved to OpenWRT on my gateway router. Prior to that, I’d just used an Airport Express, and several other Extreme and Express units around my house.
I still use the Express (and Extreme) units for WiFi. Until yesterday, I’d repurposed a couple of older Airport Express units that were running the IoT network themselves. I could have had those as an isolated network, but it’s convenient to be able to have some connection between the two. I do use a set of firewall rules to allow devices on LAN to access IOT, but not vice-versa. This allows me to update the firmware on my esphome devices without having to switch WiFi networks.
So, I had used a VLAN (and made it untagged on one port of the switch in the gateway router) for IOT. Then I had this connected to an older AirPort Express, which created the network. Then, and additional (only linked wirelessly) Airport Express extended this network.
I had thought that it would be neat to have the regular WiFi access points also creating the IOT WiFi network: indeed I nearly bought more Ubiquiti APs to install here just to do that (I put them in the beach houses).
I’m glad I didn’t: it turns out the the Guest Network functionality on the Airport devices can be repurposed for what I needed.
The magic bit of information is that the Airport base stations uses VLAN untagged ethernet frames for the main WiFi network, and VLAN 1003 tagged ethernet frames for the “Guest” network.
So all I needed to do was change the VLAN id I was using for IOT, and ensure that my router (and managed switch(es)) were passing through tagged frames for VLAN 1003 to all relevant devices. And then configure the IOT network on the existing Airport base stations. Oh, and unplug the two standalone Airport Express units.
The other thing I was able to do is add virtual Ethernet Adapters on some devices so they also exist on the IOT VLAN. This turned out to be much easier on a Mac than I thought it would be thanks to Apple Support than it was on Raspberry Pi, although not by much.