DIAGNOSIS The guest network feature is not accessed via the main Wi-Fi settings tile. It is a separate, easily missed toggle within the device settings.
FIX Open the Google Home app. Tap on your primary Nest Wifi Pro router device (not the "Wi-Fi" tile). Tap the gear icon for Settings. Scroll down to "Guest network." Toggle it on. You can set a separate guest network name and password here. The network will broadcast immediately.
IF THAT DIDN'T WORK Your entire system may need a software update that enables the feature. Ensure your system is online. In the Google Home app, tap your router device > Settings (gear) > Device information. Check "Last software update." If it is more than a week old, the update may be pending. The system updates automatically; you cannot force it.
HEADS UP Devices on the guest network are isolated from your main network devices. They cannot see your smart home devices like Chromecasts or printers. This is by design.