Normally, you only enter this information once during setup. Then every subsequent time you turn on your television you should be on the Home Screen of Roku and just watch a channel/app of your choice.
What you are describing with your 3920(Premiere) with repeated Network logins is that you have lost network connection. (The Premiere has lost its connection with your 2.4gz band of your network(router). It then restablishes connection, and drops again. You should not have to be entering your password over and over though, you should just be able to hit the Connect Button, don't press Clear Password at the Network Screen. Ideally though, you shouldn't have to doing this either, the Premiere should just be going straight to Home Screen when you press power on your remote.
The Premiere is going to operate on the 2.4gz band of your network. You may be running into a congested channel or interference from your Premiere and router, which is disconnecting you from your network. From within your router settings under your 2.4gz wireless settings, try changing the assigned channel (1-11 with 1 and 11 preferred for troubleshooting), setting your protocol to g/n, and using bandwidth 20mhz.
From the Premiere's standpoint, make sure you are plugged into the wall for power, not the tv. This will provide Premiere constant power allowing it to go to sleep, but not completely cutting power off to it like some USB TV power ports.
Once you get the network configured properly, the Roku device should stop giving you headaches.
Post back with some details of your router model, your network, etc (what/how many devices etc,) what steps you have done and should be able to help you get up and running a bit more smoothly. Screenshots always help when possible if you run into problem.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
