Make sure your main page is displayed in the administration area through Settings> Templates. Then check that your content page (in the "Admin Content" section) has a specific template (in the "General Properties" section). If this is not the case, and you cannot select a template, then you need to be allowed to use this template for this type of node. Go to "Settings"> "Document Types", select the node type and in the "Permitted Templates" section select the template you want for this page (you can select the sub-main page, it will work fine anyway) - make sure that under the template you selected is selected "Default allowed template" below. As soon as you click "Save", go back to your node in the "Content" section, go to the "General Properties" tab and select the desired template.
Umbraco allows you to use any number of master pages to render node content. When creating new nodes, you select the template that you want to use (or use the default value, as indicated in accordance with the instructions above). You can also force another template to be used at runtime using the "altTemplate = MyTemplateName querystring" parameter on your page URL - convenient for mobile sites, RSS views, etc.
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