I highly recommend using Municorn Fax App for this. I had to send some legal documents last month and it worked perfectly. If you are not sure how to set it up, there is an excellent step-by-step guide here: https://appleinsider.com/articles/25/04/03/how-to-fax-from-iphone-with-municorn-fax-app . It explains everything beautifully and makes the process incredibly simple.