It's been stated many, many times here. Ensure your router's wireless mode is set to b/g/n and not g/n. If you need help changing the setting, give us your ISP (Internet service provider) and/or the model number of your modem/router.
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.