If you cannot login to backup device of Failover (HA) cluser with MFA enabled, please use recovery code for MFA login. Do not forget to save a new recovery code after a successful login.
Explanation: In Failover (HA-cluster) with enabled database replication, backup device have modems disabled. This is to avoid duplicates of sent SMS (Outbox folder is replicated from Master > Backup). When modems are disabled, SMS OTP code will not be sent. That is why recovery code must be used to login in such configuration.