Find nearby users by real world location

Hi I am looking to add a feature to my mobile game (IOS and Android) to connect to trade with users nearby in a certain radius based on the real world geolocation. Whats the best approach to find nearby users who are also running the game right now?

I have looked into using bluetooth using assets such as this: https://forum.unity.com/threads/android-bluetooth-multiplayer.188667/ but it seems like there is nothing for bluetooth multiplayer between android and IOS.

There is plenty of AR tools like this one: https://cloud.google.com/maps-platform/gaming however non seem to have the feature I am looking for.

Or perhaps I have to share and compare player locations via a game server. However I have no idea what my first steps would be for that.