Additional sign-in or account-related issues within Microsoft Teams for business
You can refer to the AI's answer first, in addition, one important point is that the available joining options are controlled by the meeting organizer’s organization. In Microsoft Teams for business, participants who are not signed in, or whose identity cannot be verified by the organizer’s organization, can be treated as anonymous/unverified participants. Whether they are permitted to join depends on the applicable Teams meeting policies.
For participants using an account from another organization/domain, External Access can also affect how Teams recognizes and allows that participant. If either organization blocks the other domain or a user-level external access policy prevents the connection, the participant may be asked to use another account or may not be able to join as an authenticated external participant.
Therefore, if you are currently unable to join without signing in or with your other-domain account, it would be helpful to check with the meeting organizer first. If the meeting belongs to your organization and the issue persists, your Teams administrator can review the anonymous meeting join and external access policies in the Teams admin center.
For more information, you can refer to:
Manage anonymous participant access to Teams meetings and events
Plan for meetings with external participants in Microsoft Teams
External participants (users) are blocked from joining a Teams meeting
I hope this information is helpful. If you have any further questions, please feel free to ask.