How to cite a journal paper which do not have a page number?
Officially that number seems to be called Article Number (see the Table of Contents of Vol. 176 of that journal; the journal seems to have switched from 'page numbers' to 'article numbers' with Vol. 166).
It nevertheless seems safe to call it the page number. The reason is that Elsevier itself deposited that 'article number' as the 'page'-metadata in CrossRef (a depository for research papers' metadata).
See here: http://api.crossref.org/works/10.1016/j.sigpro.2020.107674, where it says:
&quot;page&quot;:&quot;107674&quot;