Is the gateway connected to the network via Wi-Fi? Is Wi-Fi currently working in Sta mode? If so, you can use the ethernet cable to connect the PC to the gateway WAN interface directly. The gateway’s WAN port has an auto alias IP address: 169.254.x.x. The last two segments of this ip address correspond to the last two bytes of the MAC address. If the MAC address is: xx:xx:xx:xx:1d:01, the alias IP will be: 169.254.29.1
If the gateway is connected to the network via an Ethernet interface, you can connect the PC to the same LAN, and the alias IP is also accessible.
If you are unable to communicate with the gateway through this IP, you can look up the IP address of the gateway on the DHCP server that provides the IP address for this gateway. The hostname used by the gateway when requesting an IP address is RAK7249.