You need to log into your router. If it's one provided by your ISP, there's usually a sticker on it that shows the IP address (e.g., 192.168.1.1, 192.168.1.254, etc.) and a username and password. You'd use a browser to go to the IP address shown (e.g., http://192.168.0.1) and you'll be presented with setup options. Some ISPs have an app or another way to make changes.
If you have your own router post the make and model and someone will help, but it's generally the same procedure.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.