Dear Katie,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
Per the description shared, I would like to summarize your concern i.e., you have given access to the SharePoint Online resource of the different tenant and when you use your work email to access the different tenant resource, you are not able login because your account doesn't exist in the organization.
If my understanding is correct, since you mentioned re-synced and reset the permission link, can you try to access the shared link through InPrivate browser or Incongnito window and see the outcome?
If the same problem persists, please contact the IT admin of the tenant (from where the SharePoint Online resource is shared) and ask whether you can found as a Guest user in the Azure Active Directory users list.
If yes, ask them to remove you and re-invite you again as a Guest user and once you accept the invitation try to access the shared resource to check the result.
If not, please ask them to add you as a guest user and after accepting the invitation, try to access the shared resource.
We look forward to your response. Thanks for your cooperation.
Sincerely,
Mia | Microsoft Community Moderator