As others have said, for the overall system to work as described you need a shared database that it universally accessible.
The simplest way to do that is have a hidden URL on a web server, either one dedicated to the system or just an extra "page" on some existing web site.
The app on a device sends it's info and requests for data as a web page request (always the same page URL no matter how many users), with the data being sent as "post" parameters.
The web page code it addresses just returns the appropriate data in a suitable format (ej. JSON) rather than sending an actual html web page. Teh app gets and uses the data as required.
You write the "page" in PHP to interpret the data requests (with some form of security) and transfer data to/from a MySQL database on the web server.
You only create the PHP code and database once, then each device that accesses it with valid security has matching records added to the database table(s) by the PHP code.
It's a very common way of sharing data between multiple app users and works very well, as long as you think out the data exchange protocol carefully when setting everything up.
You must use some form of non-trivial security coding in the requests so that any random request to the web page URL is ignored, or better still returns a fake web page or a 301 redirect to a different site, so no one can mess with the "real" data storage system.
eg. as a concept, JSON encode then take a CRC32 of the data you are going to send as a request then use a fixed re-arrangement of the byte order before eg. bas64 coding and sending the request.
The PHP code reverses the sequence, unpacking base64, reversing the byte swapping and checking the CRC before accepting it as valid and extracting the data from the JSON string.
That (or whatever you actually implement) means only _your_ app, that has the correct encoding, can ever extract data from the database.
I've used similar principles in apps I've written. The server code / database needs no maintenance or interaction no matter how many users, once it's written and debugged.