SQL Server Distributed AG's Forwarder Is Not Syncing After Primary AG's Internal Failover

`ALTER AVAILABILITY GROUP [AG1] FORCE_FAILOVER_ALLOW_DATA_LOSS;`

There's your starting issue.

Forces failover of the availability group, with possible data loss, to the failover target. The failover target will take over the primary role and recover its copy of each database and bring them online as the new primary databases. On any remaining secondary replicas, every secondary database is suspended until manually resumed. When the former primary replica becomes available, it will switch to the secondary role, and its databases will become suspended secondary databases.
ALTER AVAILABILITY GROUP (Transact-SQL) - SQL Server | Microsoft Learn

What you most likely wanted to do, was the `ALTER AVAILABILITY GROUP [AG1] FAILOVER;` command.

FORCE_FAILOVER_ALLOW_DATA_LOSS is only for situations where the primary is unavailable, automatic failover has not occurred, and the regular FAILOVER command does not work.

As for why resuming doesn't work, I'll have to admit I don't personally have sufficient experience with DAGs.
I would suggest verifying whether the databases are suspended on the primary as well, and running a MODIFY on your Global primary resetting your modes.
Resetting SEEDING_MODE is how you'd restart (or cancel) automatic seeding attempts, for example.

ALTER AVAILABILITY GROUP [DAG1]
   MODIFY
   AVAILABILITY GROUP ON
      'AG1' WITH
      (
         AVAILABILITY_MODE = YourCurrentMode,
         FAILOVER_MODE = YourCurrentMode,
         SEEDING_MODE = YourCurrentMode
      ),
      'AG2' WITH
      (
         AVAILABILITY_MODE = YourCurrentMode,
         FAILOVER_MODE = YourCurrentMode,
         SEEDING_MODE = YourCurrentMode
      );
GO