Publishing in Mathematics and Computer Science related journals
The rules for Elsevier are stated on this page.  Essentially, as long as all authors of your paper work at institutions in countries in &quot;Group A&quot; then you get a full waiver.  If any author's affiliation is outside this set of countries, you don't (there is also a set of countries that get a 50% waiver).
It's unclear whether you can get a waiver if you don't have any institutional affiliation.  I suspect it may be difficult, since Elsevier is huge and the system is supposed to work automatically by detecting your country of affiliation.  If they don't even respond to an email to support, I wouldn't be optimistic.