It sounds like you will have to add MultiSubnetFailover to the application's connection string or properties https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/sql/sqlclient-support-for-high-availability-disaster-recovery. The Availability Group has this enabled by default so you shouldn't need...

If we have large databases or one that will be heavily used, then we try to put the data files on different disks. Otherwise, same disk.