You can activate a device reset by logging in with your TUM ID under https://mail.tum.de and then by selecting "Options > General > Mobile Devices" in the upper right of the page. Highlight the device and click on the erasure symbol "Wipe all data":
Screenshot: Options
Screenshot: Mobile devices
You will receive an e-mail as soon as this is initiated.
This will completely reset the device, which means all accounts and other user data and settings will be deleted from the device. This will prevent someone who finds or steals the device from gaining access to the accounts. The reset occurs as soon as the device and Exchange account are synchronized.
The new "owner" could have initiated a complete backup of the system prior to the reset, which could then be reloaded after the reset. If in the meantime you have deleted the mobile device from the list (as recommended by the system notification), the attacker can gain access to your account. Although the device re-appears in the list, the question is whether you noticed it.
Wait a couple of weeks before deleting the device from the list. If the device requests synchronization, a new reset will be initiated and you will receive a corresponding e-mail notification.
It's always a good idea to change your password in any case, which you can do via TUMonline.