Do not change the margins.
Repeat: Do not change the margins.

Instead simulate margin changes with textboxes/Frames or Paragraph Indents.

Do not use a Section Break.

• Setting Up Letter Templates - Ribbon by Suzanne Barnhill, MVP
• How can I get a different header/footer on the second page?
• Letterhead Textboxes and Styles Tutorial
• Margins and Indents - Ribbon
• When to use Section breaks in Microsoft Word (and when to not use Section Breaks)

The screenshot is from the tutorial.




In direct answer to your question...

See: What gets carried over when you start a new section?

One of the things that gets carried over is the setting to restart numbering at the start of a section.
You can go in (each) new section and use the Format Page Numbers dialog to restart.
Page Numbers in Word 2007-2021 (365)