How to create a unique identifier for tracking

I am working on an application that has a web version, a mobile web version and an ios app . I need to track the number of visits, login, etc. To do this, I need each user to have a unique tracking identifier , which cannot be user_id, as some users are guests .

The way we work is every link that we publish in order to attract new users to the application at "http://core.APP.com" and the downloaded file redirects to the backend to:

• Application store if the user is using an iPhone and does not have an application.
• The ios app if the user is using an iPhone and has an app.
• Mobile version if the user is using a mobile phone. (M.app.com)
• Web version for other occasions. (Www.app.com)

The options I learned are as follows:

• Use IP as an identifier . (This is not a good solution, because there can be many users with the same ip, and corporate network users can be an example).
• Use of cookies . (We lose user tracking if they log in by opening the iphone app directly)
• Using a hash created by calculating the md5 string created by concatenating ip and http_user_agent. (This was the best option we found, because we not only share ip, but we add http_user_agent, which should make the identifier more unique).

Is there a way to make the identifier unique to all users?

NOTE. We plan to create an Android app in the future. I would appreciate it if the answer was compatible with Android.