@vesko41 

Upon successful reset procedure ("Press and hold this button for 2 seconds until all LEDs turn off to reset the router to
its factory default settings") the C50 factory defaults are:

• The default Username: admin
• The default Password: admin
• The default IP Address: 192.168.0.1
• The default Subnet Mask: 255.255.255.0

Set your computer LAN card with a static LAN IP address 192.168.0.2/24 (No GW / No DNS).

Connect your computer to C50 LAN port and open http://192.168.0.1 in a browser to access the router's WebGUI.

If you still can't access the router WebGUI then probably it's dead and you need a replacement (under warranty or not).

There's a TFTP recovery procedure you could try as a last resort though - see details.