Guest Wi-Fi provides an isolated wireless network which is independent from your main WLAN. Guest clients have internet connectivity and restricted LAN connectivity. Guest Wi-Fi typically relies on multi-SSID. However some hardware or drivers might not support multi-SSID, e.g. Broadcom requires to use the proprietary driver. In addition, multi-SSID decreases your data bandwidth, so update the firmware or try other devices if you experience unstable network.