Need solution for IoT devices

I just moved into a new-construction condo with a lot of IoT devices (smart thermostats, electronic lock, various appliances, etc), and I have my own modem, a 4-port switch that feeds the ethernet ports around the condo, and a WiFi 6e meshnet. The meshnet has 2 bands -- a shared 2.4/5ghz and a 6ghz band. IoT devices need a dedicated 2.4ghz network, as using a shared 2.4/5ghz band makes some of them squirrely.

I thought about disabling the 5ghz option from that band, but not all of my non-IoT devices are capable of using the 6ghz band (my wife's MacBook Pro, for example, is not an M3 or later, so it can't talk to that band), so that's not really an option.

I decided to try connecting an old Asus router to another ethernet port, figuring that I could disable its 5ghz band and just use it as a 2.4ghz router. Since the IoT devices don't have to communicate with the rest of my networked devices, this seemed practical. But the Asus router wouldn't connect to the internet. I tried it in multiple ethernet ports -- even the one the meshnet's base unit was plugged into -- and nothing. I assumed it was a faulty router and decided to buy a cheap older model 2.4ghz-only router on Amazon. I plugged that in, and I got the same issue -- no internet.

I was finally able to get it to work in Bridge mode, but this means it's piggybacking on my existing meshnet, not actually using the ethernet at all, and I don't like that. It's also not working (so far) with some of my IoT devices. The smart lock connected to it right away -- but the smart thermostats are not happy with it. I feel like getting it into "router" mode would be a cleaner solution -- but that's still not working. I'm not seeing anything on forums about Xfinity blocking a second router, simply encouraging people not to do this for various reasons that probably don't apply here -- I'm not sharing a band at all (if I can get it working, I intend to disable the 2.4ghz band on my meshnet) and I don't care if devices on each network can talk to each other.

Anyone else in a similar boat? What did you do to get your IoT devices working?