Firstly, ensure that “Block remote phones using stun“ and “Block Remote Tunnel Connections“ options are disabled in your 3CX extension settings (Extension -> Options -> Restrictions), as they blocks external registrations:
Also, depending on your 3CX deployment option, you may need to allow WAN connections.
Lastly, ensure that you are entering your 3CX extension registration correctly in your Ringotel admin portal.
Note that the SIP username should be the same as the extension number. Take Authorization name and SIP Password from your 3CX extension Phone Provisioning settings.