You can use the AG for DR - that is what it is designed for...but to do that you need to be able to move the file share witness to the DR site to allow for a quorum.
You would normally setup the cluster with 3 votes for quorum - the primary and secondary nodes in the primary DC and the file share. The DR node will normally have a node weight of 0 so it doesn't participate in quorum to determine cluster health.
For a failover - you need to move the file share witness to the DR DC and change the node weight on the DR secondary and manually migrate the listener to the DR secondary.