SQL Server 2017 Distributed Availability Groups
After DC 1 is down, since your secondary availability group [distributedag] is asychronous and is in manual failover mode, it is still in secondary role, so you will need to connect to [distributedag] listener and execute command:

USE master
ALTER AVAILABILITY GROUP [distributedag] FORCE_FAILOVER_ALLOW_DATA_LOSS


This will force your secondary AG at DC2 to become primary, and bring it into a read-write state