but this doesn’t seem possible even when unchecking the boxes for “same content”. HELP
A header is a property of page styles: there is only one header per page style. Therefore, you need one page style per page, which is usually not practical with a moderately long document.
However, if your header content is related to some identifiable data in the page, you might try to design a “dynamic” header with field insertions. “Identifiable” items are titles (Heading x paragraph titles), bookmarks, variables, …
Can you better describe what you want to do?
Same content refers to left/right page styles to quickly set up both. It is a “quick-and-dirty” way of creating a duplex document. IMHO, the best way is to design explicitly first, left and right page styles to accurately control all parameters.
If this helps, please tick the checkmark at top left of this answer.
It is unlikely, that the header is really total different. Likely you need a field in the header, which adapts itself to the content of the page. Common fields in headers are “Page number” and “Chapter”. Inserting a Chapter Name and Number in a Header or a Footer - LibreOffice Help