IOS, which ready-made chat platform to choose?

Question

iOS

The task is to write a chat using some layer of business logic. Chat clients should work under web and IOS.
Requirements:
- users are brought into the system by administrators
- users (or administrators) can communicate in a general chat (everyone sees everyone) or in private chats with administrators
- text and file exchange must be supported
- message history must be stored on the chat server (that is, can be stored somewhere else, but on the server - a must)
- flexible API for adding to the business logic (well, actually, like everywhere else)
- reliable and fast transport (messages should not be lost, come in the wrong order, the exchange should happen fast enough)
In this case, writing all this from scratch is impractical, ready-made platforms exist and offer their services.
Tried Quickblox. There are many possibilities, the API is rich, there is an SDK for many platforms. But it was unpleasantly surprising that, in general, the basic application had to be written by asking about 10 questions to the support (which were not answered either in the documentation, or in the examples, or often in the support itself). The speed so far also leaves much to be desired - perhaps this is due to not the maximum tariff, but perhaps this will continue.
What analogues have you tried, what did you like, what not?

Answer 1

For those who will use the search in the distant future - https://layer.com
At the moment you need to apply for Early Access.