Since we cannot create a custom Administrator role where we can add the Mobile Device Access permission to, a workaround that can be done to let NetSuite for iPhone app users login with an Administrator or Full Access role is to add the Mobile Device Access permission to the employee record through Global Permissions.
Before doing so, make sure that the feature is enabled in:
Setup > Company > Enable Features > Employees > Permissions > Global Permissions
Once the feature is enabled, you can now Edit the employee record then navigate to Access > Global Permissions, add the Mobile Device Access permission and Save the record.
Note: Adding the Mobile Device Access permission to a user's global permission means that no matter what role he uses to login on the app, he will still be able to login since global permissions always take precedence over the user's individual role permissions.