Guest Wi-Fi never was 2.4ghz only, that's a myth.
You can't split out 2.4ghz, it will always be combined into one SSID. Both for the primary network, and the guest network. I'll paste in my usual reply.
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Things to make sure, before trying various workarounds:
• Make sure that the IoT device(s) in question supports the amount and type of characters used in your chosen network name (SSID) and password; some IoT devices might have limitations here. In general avoid anything but a-z in the network name. I've heard of devices not supporting whitespace in network name (SSID) or password for instance.
• Some IoT devices have alternative set up procedures if the regular way does not work, which could help.
Other workarounds that might work:
1. Temp additional 2.4ghz only access point/hotspot, using the exact same wireless network name (SSID) and password as the Nest Wifi.
First turn off your Nest Wifi network by removing the power. Then:
• Using two phones: Use one of the phones to broadcast a 2.4ghz only hotspot, and the other phone to setup IoT devices to it. 1st phone must be connected to internet somehow (SIM card), 2nd phone must connect to the wireless network broadcasted by the 1st phone.
• Using a PC: Feed your laptop (Windows or Mac) an internet connection, either by ethernet cable from your modem, or by mobile network tethering + USB. Then use your laptop to host a 2.4ghz only Wi-Fi.
Then get your IoT devices connected to the temp hotspot/access point. After that, disable temp hotspot/access point and plug in Nest Wifi, and the IoT devices should jump to the Wi-Fi provided by Nest Wifi.
2. Use an old iOS, iPadOS or Android device that only supports 2.4ghz to set up the IoT device. Must be new enough to support the Google Home app though.
3. Force a 2.4ghz only connection by walking far enough from the Nest Wifi puck(s) to make sure your phone/ whatever device used for setting up the device in question connects to the 2.4ghz network, then start setting up the device.
4. Permanent additional 2.4ghz only access point, different SSID than Nest Wifi.
Buy a cheap access point or repurpose a wireless router or range extender to broadcast a 2.4ghz only SSID with a different name than your Nest Wifi network name. Connect it by ethernet cable to the primary/main/master Nest Wifi puck preferably. Add more ethernet ports by using an unmanaged switch if need be ( https://support.google.com/googlenest/answer/6274262?hl=en ). Make sure this additional Wi-Fi broadcastong device works as an access point only, else you might not be able to connect to the device in question from a device connected yo your Nest Wifi network.
I don't work for Google.
