When you send a fax out from your app, the best way to track the fax status and read the fax history is to call the message-store or the call-log endpoints.
If you sent a fax out from your own app, that fax status or history data will not be shown on the RingCentral Phone App under the Fax list. In your question, you mentioned the RC App, not sure if you referred to the RingCentral Phone app or not but I don't see the RC App (previously named Glip) has a Fax list.
You also mentioned that no error on the Developer Portal. This is correct as the Dev Portal reports only API call status (200 or 4xx etc.). Errors here do not reflex if the fax was sent successfully or the recipient could or could not receive a fax.
So, all you should do is to read the call log or the message-store to track your fax activity
