@Nazrul-Azraan_ For TP-Link routers, you could press and hold reset button until power LED flashing to reset the device to factory defaults.

@AlexAloha You could refer to this guide to configure the parental control, add all users and block the site for them.

@vverdura Try factory reset the router by holding reset button for about 10s to see if there is any response. Or try the firmware recovery again.

@Issak You mean all wired and wireless devices connected to the router has the low speed, or only wireless devices? If only wireless devices slow, you could try to change Wi-Fi channel and channel...

@christianh It is normal that VPN client IP address is in different subnets from the LAN IP, but VPN Client will still be able to access the LAN network, you may have a try.

@Shutterman Make sure you are connected to the C50 WiFi and turn off mobile data or VPN, restart the App and router. By the way, it seems that TP-Link has retailer version and ISP customized version...