There are many years of discussions about faxing via VoIP. The bottom line is that it is unreliable at best, and it can be frustrating to get it to work at all. There is no silver bullet, despite various posts about various arcane settings changes.
Given that you're already limiting it to the recommended maximum speed, the other setting that makes a big difference is to turn OFF the fax machine's error correction. EC on fax is very primitive; all it does is tell the sending machine to resend the page. It's unforgiving. If it can't successfully send the page, it will just give up and hang up. With EC off, you will probably get some messes on some pages (stripes, garbled characters, etc), but at least it should send.
Bottom line: simply avoid sending faxes that way. Instead, use a email-to-fax gateway, or a service that allows you to upload a PDF or JPG file to send. Their fax servers will handle the actual fax communication. You can also receive faxes that way, far more reliably. Of course, you can also scan and email the document as an attachment, or use a file sharing service like Google Drive, Microsoft Onedrive, or Dropbox.