A header or footer (where you insert the field for page number) is a property of a page style. A page style can have only a single header and a single footer.
If you need pages with different header/footer, you must use different page styles.
Keep Default Page Style for the main topic of your document. Create a user page style for your first part. Give it the same paper size and orientation. See if margins should be same and don’t activate header nor footer (unless you put there something else than just a page number).
The boundary between the first part and the discourse part is a special page break. It is inserted by Insert
>More Breaks
>Manual Break
so that you have access to the dialog configuring the next page style and starting page number.
Of course, this will work reliably only if you save native (*.odt) and not .doc(x).
In case you’re not familiar with styles, read the Writer Guide for an introduction.
PS: @provlima1
Don’t post the same question at 10 days interval. It is fortunate that this site is consistent and you get the same answer.
If something was not clear in the first answer, it is better, to avoid site clutter, to request clarification through a comment.