If it can be used by directly connecting the network cable to the computer, it means that the incoming network cable has been connected to the router. Please disable the DHCP function of the current router and set the IP address to 192.168.1.2XX (XX is any IP address, as long as it does not conflict with the IP address on the network). Connect both your computer and your incoming router to the LAN port and you'll be able to use it normally. In this state, the router can be used as a wireless switch.