You can try a factory reset from the recovery menu. To get into recovery mode, power on your phone while holding the Volume Up button.
You'll get a little screen with some options. Use the volume rocker to highlight "Recovery" and select it with the Power button.
Now you'll have a screen with a triangle and an exclamation mark. Hold the Power button and press Volume Up to bring up the recovery menu options.
Select the option that says "Data Wipe/Factory Reset"
This will return your phone to factory condition, meaning all apps and settings will be removed. Items in your internal storage won't be touched though.
It may or may not help, though. But since you've only had the phone a few days, it shouldn't be too much trouble to go through the setup process again. Also, an update is supposed to come out in the next few weeks. That could fix your signal issues.