SELECT log_reuse_wait_desc
FROM sys.databases
WHERE name = 'Main DB Name'

node        log_reuse_wait_desc
------------------------------------------------------------
primary     AVAILABILITY_REPLICA
Secondary-1 LOG_BACKUP
Secondary-2 AVAILABILITY_REPLICA

-- On SECONDARY 1
   ALTER DATABASE DbName SET HADR OFF;

-- On SECONDARY 1
   ALTER DATABASE DbName SET HADR AVAILABILITY GROUP = AG_NAME;