うまくいくかわかりませんが・・・・ 参考のようなので、Teams Freeでどうかわかりませんが、
Microsoft 365 appsでサインインしたあと、Teams(最新版で)でサインインしてみるとどうでしょう。
<参考> Sign in to Microsoft Teams
https://docs.microsoft.com/en-us/microsoftteams/sign-in-teams
~引用~
・・・・・・
Teams on iOS and Android users Upon sign in, mobile users will see a list of all the Microsoft 365 accounts that are either currently signed-in or were previously signed-in on their device. Users can tap on any of the accounts to sign in. There are two scenarios for mobile sign in:
If the selected account is currently signed in to other Office 365 or Microsoft 365 apps, then the user will be taken straight to Teams. There's no need for the user to enter their credentials.
If user isn't signed in to their Microsoft 365 account anywhere else, they will be asked to provide single-factor or multifactor authentication (SFA or MFA), depending on what your organization has configured for mobile sign-in policies.
Note
For users to experience the sign on experience as described in this section, their devices must be running Teams for iOS version 2.0.13 (build 2020061704) or later, or Teams for Android version 1416/1.0.0.2020061702 or later.