I have two Rokus in upstairs bedrooms. One is no longer connecting to the internet, tells me that the signal strength is weak. I have tried everything to get it to work, and nothing. I know it's not the wifi since, the other Roku in the bedroom next door is working properly.
Device type: Roku Premiere
Serial number: YH008Y813552
Where is your router positioned? What model is the router and how old is it? If you have an older router, you may need to get a wi-fi extender or update to a newer router in order to get a good signal to your device. It sounds like you may have a dead spot for your wireless network in your home.
Try swapping the devices. (place the weak one in the location getting better signal and see if any improvement).
While I haven't had any issues with my 3921 Premiere+ devices, this forum is full of 3920/3921 Premiere complaints. (since only one antenna in this device, once it fails/begins to fail it is going to be hard to achieve a stable connection).
There used to be a way to test the antenna hardware but unfortunately was disabled by Roku so now user is limited to moving device around to different locations to see if any improvement in signal acquisition.