I’m the PM that owns DTR, and I apologize for the inconvenience.
The commercially supported engine upgrades for DTR are done through your OS’s package manager from public Docker repositories, so this upgrade should be available in any cloud environment that has Internet access. Have you checked your security groups to make sure you aren’t blocking the engine upgrade?
Additionally, are you sure your package manager is pointing to the right Docker repositories? If you use public repositories that come by default on your OS, this may not upgrade your engine correctly. Please refer to the upgrade instructions here: https://docs.docker.com/docker-trusted-registry/install/upgrade
DTR 1.4 does work on cs engine 1.6 although this is not recommended. Hopefully, this resolves your issue, but if not, can you please file a ticket with support? I’ll track the ticket and have someone dig into this on my end. We take upgrade failure very seriously so I appreciate the notification here.