No matter how many times I update my PIN to add channels via mobile setup, every time I go into Roku to add a channel it replies ‘invalid PIN’. So **bleep** frustrating!!
If you are sure you are changing the PIN to the correct linked Roku account, try performing both a manual System Update and a System Restart under the Roku Settings.
Settings..System...Advanced System Settings...System Update..Check Now
Settings..System...System Restart
Check your Roku via Roku menu path Settings > System > About. The email address shown is associated with the Roku account to which your Roku device is linked. Is it the same email address as you are using to log into your Roku account? If not you're changing the PIN in a different account than the one your Roku is using.