I am changing from Word to Pages. I have created my custom template with all my styles etc and that is what comes up when I go for a New Document. Fine. How do I get it to use the same Custom Template when I use Pages to open a Word document?

You can't open the Word document into a template. The Word document will be converted to a Pages document and retains as much as possible as it was created in Word.

The template is a document in itself, it is not applied to an existing document whether it is a Pages document or a Word document converted to a Pages document.

You would need to either copy and paste content, using existing styles, or apply the styles to the converted Word document.

You can Import the Styles from an existing document and those imported Styles can be used to override the current document's styles:

Menu > Format > Import Styles

The process is simplified if the styles use the same names, otherwise you will need to delete the style you don't want and replace it with the one that you do want when asked, then the substitution is pretty straightforward.

Peter