GL-X750 Extender Mode not persistant and dropping connection

Hi, I have difficulties using the GL-X750 router as a WiFi extender.
I have a 4G Wi-Fi network (router A). For security reasons, I want to extend the network of router A through a GL.inet router (router B) and connect a wired IP camera to router B.

I use the Wi-Fi extender mode on router B and enter the SSID of the 4G Wi-Fi network to connect to the network. The IP address of the IP camera will be assigned by router A. It was working fine for two or three days.
However, later I discovered that the GL.inet router could no longer connect to the Internet, and the original IPs of the camera and router B vanished from the client list of router A. It worked for a few days when I reset it back to router mode and set it up again.

I have been dealing with this issue for nearly a month and please help me !!!
I’m unfamiliar with networking, if any additional information is needed I will try my best to cooperate.