If you log into your account on service.ringcentral.com and click the Messages tab, then Sent Items, you'll see a list of your sent faxes. On the right side of the page, you'll also see an option to Forward the fax.
You can also do this through the Desktop App. If you Click the messages (envelope) icon, and the Filter icon, you'll see an option to display Sent faxes.
When you click on a previously sent fax, the window will expand and expose an option to forward the fax.