Lansweeper uses the network connection of the server it is installed on. There is no function to add a DNS server on the web console.
If the target computer is pingable from your Lansweeper server with it's IP address, you could add the IP ranges of other network parts under
Configuration\Scanning methods, section
IP Address Range Scanning. This will work if required
ports are open between your Lansweeper server and the computers in those networks.
If there is no network connection available or the necessary ports can't be opened, we recommend that you use LsPush. This is a small executable which scans the computer and sends the result back to your Lansweeper server using direct server connection (port 9524/TCP by default), file share or mail. You can find details
here. It is possible to configure a scheduled task for running LsPush and deploy it via group policy.