Your problem may have been caused by an update to your router which affects some Roku models. The update changes the router's wifi connection parameters for the 2.4 GHz band from b/g/n to g/n. This has been reported with equipment from Comcast/Xfinity, Cox, and more recently AT&T, but there may be others.
Affected users report the Roku identifies the local network as available and rates the connection quality, but will not then connect.
The fix is to switch the router 2.4 GHz configuration protocols from the g/n wifi set by the update back to b/g/n.
If you don't have access to your router's configuration controls or just don't know how to do this you may have to have your internet provider do it for you.
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