Some online journals give an article number but no issue number or page numbers (beyond that of the pages of the PDF).
Format: in text citation
This is explored further... (Surname, Year, p.).
Format: reference
Author surname, initials. (Year) Title of article. Journal Title. Volume number(article number), pp.first and last page or line numbers.
Example: in text citation
This is explored further... (Jones, 2018, pp.1-11).
Example: reference
Jones, A. (2018) Journal articles. Journal of Learning. 12(article 1281), pp.1-11.