Choosing the 2.4 band on my iPhone with a Mesh Router

The Orbi is broadcasting what is known as a simultaneous dual band 2.4 GHz and 5 GHz signal using the same wireless network name for both bands.

Since this is the case, you won't be able to tell any Wi-Fi device to choose one band or the other unless your router has the options and capabilities to do this. In other words, you would set up the 2.4 GHz band to use one wireless network name and the 5 GHz band to use a different wireless network name.

Unless something has changed very recently, Netgear / Orbi does not support the features or options that you need.

Since 5 GHz signals are weaker than 2.4 GHz signals, you might try connecting the iPhone at a more distant location to see if that helps. The 2.4 GHz signal will be stronger at a distant location and the iPhone should choose the stronger signal with which to connect. That's the theory anyway.

Not sure why you need the iPhone to connect at 2.4 GHz, but if you can do whatever you are trying to do with the iPhone connected to the Orbi signal, and things work OK, it does not really matter whether the phone is connected at 2.4 GHz or 5 GHz.

I told their tech support that I was confident the Orbi was choosing the correct band because it always has before, including on a similar item.

Unfortunately, this is not correct. The Orbi....or any other router for that matter....broadcasts the same signal everywhere. It is up to the Wi-Fi device to select and choose the frequency....2.4 GHz or 5 GHz....with which the device will connect.

For example, my Mac laptop will connect at 5 GHz if it is in relatively close range to the router, but if I move the laptop a few rooms away from the router, it will connect at 2.4 GHz, since those signals are stronger than 5 GHz signals.

A 2.4 GHz only Wi-Fi device is supposed to be able to connect to the 2.4 GHz signal on a 2.4 GHz and 5 GHz dual band network, but not all do. In cases like this, your only option is to create a 2.4 GHz only network....and the Netgear Orbi cannot do this.

They told me the only other thing they know for me to try is to be sure the iPhone I'm using for setup was also on 2.4.

Well, this sounds good, but the only way that you can force an iPhone....or any other Wi-Fi device....to connect to 2.4 GHz when both 2.4 GHz and 5 GHz signals are present is set up the 2.4 GHz signal with one name and the 5 GHz signal with another different name. Then, you can "point" the iPhone or Wi-Fi device at the name of the network that you want to use.

That's why I suggested that you try to connect the iPhone and thermostat at a distant location from the router....because 5 GHz signals are weaker than 2.4 GHz signals, there is better chance of the stronger 2.4 GHz signal reaching the more distant area.

Frankly a 2.4 GHz only device is using very old technology.

Sometimes, a 2.4 GHz only device is simply not compatible with a dual band network. It should be, but as I hinted before, should is not the same word as will.

You may have to consider a different brand of thermostat, as I doubt that you want to think about changing out the mesh network.