@Okkk
Does it mean there are 2 X Archer C50s, one is working as the main router, and the other is extending the wireless network of the main one? If that is the case, please first check and ensure you have a stable wireless connection on the main Archer C50, you can test it by connecting a laptop or phone to the wireless network of the main C50, and monitor for some time.
If the wireless connection of the main C50 is pretty stable, and other laptops or phones can connect to it and obtain IP address properly, but only the 2nd C50 loses the connection, try to move it closer to the main.
If the issue persists, try to fix the wireless channel on the main to a specific one, you can run a Wi-Fi analyzer to review the wireless channels around your network, choose one with less interference, then reboot both the main and the extender to monitor the connectivity.