How to associate an iPhone with an Apple ID/account via iTunes?
Maybe I'm missing something from your question, but...


Launch the Settings app on your iPhone 7 Plus.
At the very top you'll see your photo or an icon pertaining to your name. Tap on this. 
On this screen you should see a whole range of settings, including for iCloud, iTunes &amp; App Store, etc. You should also see your devices listed.
Ensure that you're logged into your Apple ID for iTunes etc
Assuming you see your iPhone 7 Plus in the list of devices, tap on that as well to verify the device. 
If prompted to sign in to iCloud etc, then enter your password and tap OK.
If prompted to answer security questions, etc, then do so.


After going through the above process, do you find your iPhone 7 Plus is associated with your account? If not, then try downloading a song or app you've previously purchased with your Apple ID and check again.