Here are a couple of other things to try. They may not be universally successful, but they are at least worth a try.
Restart your router, even if other devices on the network are not having problems. When your router has completed its startup, try repeating your Roku setup.
Try using a different WiFi network to do the update - perhaps via a WiFi hotspot on your mobile phone or by taking it to a friend's house - and see if it will complete the startup. If successful, then redo the WiFi connection to use your intended WiFi network via Settings > Network > Set up connection. Why this works, when it does, I haven't a clue.