Subdomain tracking with Tag Manager

I’m trying to have all the different pages across a domain tracked with a single tag. I have set up the tag for the main URL, but am not sure if this allows for tracking across all the pages on the domain. In Debug-view, the preview is abandoned when I navigate to another page other than the main page, which I assume means that it’s not tracking.

… Because when you navigate to another page, the mtmPreviewMode HTTP GET parameter disappears…
→ Reload the page adding the mtmPreviewMode HTTP GET parameter.
If you still don’t have the preview, are you sure the JavaScript bootstrap code is present in all pages?