There are a few complications here.
The first complication is that you want to redirect an hostname which is using HTTPs. In fact, normally you could use the DNSimple URL record, but it doesn't work with https as you would need to install the SSL certificate on DNSimple servers (HTTP redirect header is sent after the HTTPS negotiation).
One option would be to redirect at server level, but since your app is hosted on Heroku you have no control over the server configuration.
Therefore, in your case the only option is to point the various domains to the same Heroku application, add the domains as custom domains to your Heroku app and handle the redirect internally. That means your app should check whether a request comes from one of the alternate domain and if that's the case it redirects to the canonical domain.
There are several ways to check it in a Rails app. The most effective is probably using a custom middleware or the Rails router file, so that the request is handled as soon as possible and it doesn't build an entire request stack in the app, as it would happen for instance with a before_action.
On the DNSimple side, you will just have to create an ALIAS record (if the hostname you want to resolve is an apex domain) or a CNAME record (if the hostname is a subdomain) that points to the same Heroku SSL endpoint of your application.
However, the last complication is that Heroku doesn't allow multiple certificates for the same app. If the domains you want to redirect to the canonical one belongs to the same name (e.g. are subdomains) you can just purchase a wildcard certificate. Otherwise, if you want to cover multiple different domains in the redirect, you will need a multi-domain SSL certificate.
