The Roku Express is a single band WiFi device, so make sure your 2.4 GHz radio is enabled on your router. If your Internet is through a cable company, some have been reported to have disabled the 2.4 radio on users' routers. Also make sure the channel in use is between 1 and 11. If the router is on channel 12, 13 or 14, Roku devices won't see it. Again, if your ISP provides your router, you might not be able to change it yourself, and you'll have to ask your ISP tech support to make the changes.
Dan
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, just another user.