I cut the cord for rip-off Cox cable tv a year and a half ago and bought three Rokus. All was fine until my Roku Premiere would not establish a network connection and then a few days ago it happened to my Roku Premiere Plus. When I tried a different Roku each time it connected immediately. Fortunately I bought a SquareTrade warranty with each Roku. When I put in a claim on each, they sent me, and are sending me a check for what each cost.
With SquareTrade, the four year warranty cost $2-4 each but you have to record the serial number and submit your Roku receipt - a PITA but well worth it. Conclusion: Rokus are great but do-not work forever, so I would recommend an extended warranty.. They do get hot and this probably fries their innards.
Your Premiere models only work in the 2.4 GHz wifi band. Your problems with these models may have been caused by an update to your wifi router.
There are some router setting issues that can prevent establishing Roku wifi connections. And to make matters worse, some of them can be caused by recently-reported router updates. These have primarily been reported with routers supplied by internet providers (Comcast/xfinity, Cox, and AT&T so far), but it's possible they could also occur with other internet providers or user-owned routers that have received similar updates.
If you need to make configuration changes on equipment supplied by your internet provider but you have no access to its settings, you may need to request your internet provider to make the changes for you.
If your problem was not caused by any of the above, often a restart of the router (even if other devices are still connecting) followed by a restart of the Roku will allow the connection to be reestablished. Depending on your Roku model and Roku version number you'll find the "System restart" option under either "Settings > System > Power" or "Settings > System", or if more convenient you can just remove the Roku's power for a few seconds and let it restart.
@makaiguy, my Roku Express. works fine where my Roku Premiere failed to connect. I have a dual router for 2.4 and 5.0 GHZ, so I don’t think that was the issue. I did resets/restarts you outlined, etc. before I gave up on the unit. I have two Roku Stick+ to work with the 5.0.
Thanks for your input.