Delphi FMX Client Server (Internet)

I would like to have a simple chat app written in Dephi 10.3.3 Community Edition with a Server (running on Windows 32/64, so maybe even in VCL if not available in FMX) and a bunch of Clients (running on Android and Windows ..and maybe even Mac OS and iOS in the future, but for now only Android and Windows) which exchange text and files over the internet (not only LAN) with the server handling some kind of database. So it should work like usual texting apps like WhatsApp or any other chat app. I searched the whole Internet, but I didnt find a good solution for me to do a simple Client-Server App in FMX. I did find the Sockets (TServerSocket and TClientSocket), but as they only run on Windows I can't use them. I also thought about setting up my own REST Server, but I don't know how to do it or whether its really neccessary for my Chat App. So how do I implement a simple Client-Server connection in cross-platform fmx app across the Internet?

I already have tried the following code: TCP/IP Indy Chat Delphi 10.3.3 FMX (using internet, not only LAN) but I had no success running it across the internet so far as it just works on local area networks..