There does not appear to have been a reply to this question, and I still grapple with the same issue (iOS 13.4, Numbers 10.0).
There is a workaround for those with a Mac:
Create a new document in Numbers
Change the Print Margins (File > Print > scroll to bottom of Print Setup and change Page Margins)
Go to File > Save as Template > Add to Template Chooser > type file name
Right-click newly created Template > Share > Airdrop > choose your iPad
On the iPad:
click on “Add to Template Chooser?”
The newly created Template with your custom print margins is now available on your iPad in Numbers.