Jonathan, are you trying to login to the Sandbox version of the online account portal? https://service.devtest.ringcentral.com (note the "devtest." which is added to the normal online account portal URI to switch to the Sandbox version)
If you login to the Developer Portal (https://developers.ringcentral.com) -> My Account -> Sandbox Accounts you can get the appropriate Main Company number you will use to authenticate into the Sandbox version of the online account portal (see URI above).
If that doesn't work, and you still don't receive the email for password change, let me know.