Hello Trimarchi Fabio, Since the device registered for second-factor authentication are not getting any MFA prompts, you are unable to complete the authentication.
In such scenarios, another Global Administrator can help to revoke and re-register MFA on a mobile device. However, if you are only the Global Admin in the tenant, this qualifies as a tenant lockout scenario. To resolve this, we need to engage the Data Protection team via a support ticket to unblock your access.