I want to build an IoT mobile app(iOS/Android) that will mainly do the following: user management(login/subscribe new user);add/configure new IoT device; interact with existing devices. Interact with IoT devices is the easiest part for me. What I'm struggling is the first 2 items. For this I was thinking to use a cloud platform like AWS(https://aws.amazon.com/),Azure IOT(azure.microsoft.com) or WATSON IBM(https://www.ibm.com/watson/). Reading the docs of them I could not see how to perform the first task or how to register a new device via a mobile app. Any comments are more than welcome.

Also I was thinking building a database on a server that will manage the users, and sending/receiving messages from the IoT device/app as this app that I'm trying to build is not so data hungry(reading temperature and interacting with 2 relays)

P.S. I know about Blynk.io but the prices/conditions that they are offering is exceeding my budget.