You followed the steps, but the guest network still refuses to work. Here are the three fixes that solve 99% of Xiaomi AX1800 guest network issues.
If you cannot find the Guest Network setting: create guest network on xiaomi mi router ax1800 work
Symptom: A smart plug or cheap IoT camera sees the guest SSID but fails to connect. Fix: Xiaomi routers often force "WPA3" encryption on 5 GHz by default. Old IoT devices only support WPA2 or even WEP. Go into your Guest Network settings and manually change the Security mode to WPA2-PSK (AES) only. Also, connect the IoT device specifically to the 2.4 GHz band. You followed the steps, but the guest network