@Hett Hello,
Here is the latest firmware for AP300, please confirm the firmware is up to date:
https://www.tp-link.com/support/download/ap300/#Firmware
May I know when cell phone connects to AP's SSID, did it ask for wireless password? After you put in WiFi password, what is the Error message?
Please share a photo if possible, thank you.
If cell phone does not ask for a wifi password but report an Error directly , please try to forget WIFi on your cell phone and reconnect :
Go to Settings-> Wi-Fi on your phone, Tap (or Long press the router's WiFi name ->Forget / Delete the network
What is the SSID of your PC has connected to and what is the IP & gateway on your PC when connected to AP‘s WiFi:
https://www.tp-link.com/support/faq/838/