You cannot use a .dev or .app domain/subdomain on Remixer Cloud Hosting since they do not support HTTPS/SSL certificates. This applies to situations when you publish Remixer Cloud-hosted sites using the Publish button (not the Export to WP button feature, which exports to DreamHost's shared WordPress hosting).
The following describes why HTTPS is forced by default on both .dev and .app domains.
Why do .dev and .app domains force HTTPS by default?
The .dev and .app domains are both owned by Google. As part of Google's initiative to secure website connections, .dev and .app domains force HTTPS by default.
For example, you can see that this is mentioned when you purchase a .dev domain registration:
Since HTTPS is forced at the registry level by Google, there is no way to override this redirect. The only option is to install an SSL certificate to ensure your site does not throw a browser warning.