You can configure `EXM.RendererUrl` in the `Sitecore.EmailExperience.Core.config` config file.

In case the setting is empty, the `RendererUrl` is resolved on the first request to the application. In the `<httpRequestBegin>` pipeline, there is a `Sitecore.Modules.EmailCampaign.Core.Pipelines.HttpRequestBegin.OnHttpRequestBegin` processor that that does that:

    GlobalSettings.RendererUrl = WebUtil.GetServerUrl(args.HttpContext.Request.Url, false);

You said that it worked ok previously on HTTP, but after you switched to HTTPS, it does not work. First I would focus on fixing the SSL/TLS issue. I bet that there is some problem with the certificate.