From here: https://www.access-programmers.co.uk/forums/threads/subreport-headers.256308/
In the Subreport do the following and you will have the Report header on subsequent pages.

1. Click on "Add a Group"
2. Select "Expression"
3. When the Expression Window appears, type in "=0" and click OK
4. Make this group only have a header section
5. Click on the group's header section
6. Go to Property Sheet and make sure "Repeat Section" is set to "Yes"
Done!