If your current podcast hosting does not allow you to redirect your podcast feed to RSS.com you have to manually change the podcast feed URL in Podcasts Connect.
1. Login https://podcastsconnect.apple.com
2. Click "Podcasts"
3. Click the "Show" you want to update
4. Inside the tab "Show information" click on "Edit" inside the RSS Feed section
5. Add the RSS.com feed URL and click the "Save" button