So after 12 hours of sleeping, it seems as though the issue was DNS / Sandbox propagation. After waiting 12 hours, the authentication requests succeed every time now. Once again this is a very common issue with new Sandboxes and Authentication I have seen many times.
For creating a user account - you can call an out of the box REST API, it sounds like you're walking down this path. While you can (for example) create a single User object (record) in one REST call via an out of the box API, more than one record of one or more object type may need to be created for a user to be fully provisioned. You can use a Composite ...
Here's the documentation on How to set up Two Factor Authentication for Android and iOs Devices Set Up Two Factor Authentication for mobile (Android/iOs)
Steps to set up Two Factor Authentication for the mobile app Users
Steps in Lightning Experience:
Navigate to Setup | Apps | Connected Apps | Manage Connected Apps |
Salesforce for iOS.
You can create a Site and add a redirect from a Site URL to a destination. The destination can be any relative or absolute URL, including a MyDomain URL.
Out of the box, the hostname of the site will be auto-generated and added to a DNS for you by Salesforce. You have an option of using your own, custom domain controlled by your own DNS instead of having ...