If all other devices function properly on the network then I can be isolated to three things:
1. Your Roku’s settings may not be adjusted to your network. Most devices have the ability to reset network settings. Otherwise a factory reset could resolve your issue.
2. Your network and router may not be adjusted properly to accept incoming connection from your Roku, generally resetting your router or adjusting the settings by signing into it may resolve this issue.
3. The WLAN card on your Roku could be faulty. Most devices have a warranty to cover this in the first year or so. However, if your Roku isn’t under warranty, you may need to replace it. Generally speaking, replacing the whole unit would be better suited for a device as cheap as a Roku