QuickBlox offers a starter tier which you can use absolutely free. All you need is register a free QuickBlox account and add your App there.
Each tier has particular server configuration. E.g. with free tier you can have 20/s chat messages, 1000 concurrent users, 20/s notifications etc (check details).
If you feel that free tier is not enough for you, you can upgrade to bigger plan any time. Besides if you are looking for good quality and support you should pay attention to Pro and Enterprise plans.