Once you have inserted and formatted a citation, you should not edit it directly (even though you are able to do so). Direct edits are lost the next time EndNote formats the bibliography.
You would need to use the Edit Citation command to add a page number after the citation is added to a footnote.
Click on the citation you wish to change.
From the EndNote tab, select Edit & Manage Citations.
Locate the Pages box: Enter page numbers here.
Click OK.
If you need additional help please check out knowledege base at
https://support.clarivate.com/Endnote/s/?language=en_US
or
Contact Technical Support directly
1-800-336-4474 option 4 then 1