Apps How to detect unique devices on which user has installed app

I am working on a mobile app. The user will have web as well as mobile app access to the system. When the user installs the app on different device like Android mobile and iPhone for the same account --> how to differentiate these ? like is there any user-agent field that I can use to differentiate the devices on the backend server.

I only want the user to use apps on phones which have been verified by OTP (One Time Passowrd) using SMS.