I've just delivered a similar set up for a customer: most users of the system start by navigating to a React app. This app is hosted on AWS. You could take a look at Amazon's Static Website Hosting. The React app has a number of DeepLinks to a Mendix app, which is hosted on the standard Mendix cloud. This isn't hard to set up.
The real problem in this scenario is authentication: if users need to be authenticated to access your React landing page, need to be authenticated for your Mendix app, and you want to preserve their session you need to do some work to make that happen. We've handled that by using a single sign-on solution.
Update: I figured out a way to fix this. Because it was an on premise application, I was able to manipulate IIS and using the Rewrite URL component.
Thanks for your help.