I was in that same boat. Needed to fax a signed doc and didn’t want any extra steps. This article helped a lot: https://www.slashgear.com/1762462/municorn-android-fax-app-faxes-smartphone-mobile-simple/. Municorn turned out to be a gem. Opened it, snapped a pic of the doc, typed the number, hit send — done. No sign-ups, no weird ads. Got the job done in minutes. Super efficient for last-minute needs.
Do people or even businesses still make use of fax? I think the use of emails have replaced sending fax for a long time now? I have actually forgotten how to use a fax machine.