SQL Server Enterprise Manager Could Not Start The Service 'MSDTC'

SQL Server Enterprise Manager could not start the service 'MSDTC' on server
'NTAS21'.

1722 - (The RPC server is unavailable)

I Setup A publication on a server called NTAS4D, then Pushed The
Subscription to another server called NTAS21. If I configure so that the
changes at the subscriber are not replicated to the publisher, all works ok,
I make a change at the publisher, it is replicated to the subscriber.

If I configure for updating at the subscriber, I am presented with a screen
stating that the following services must be started for replication to work:

SQL Server Agent on NTAS4D - Running
MSDTC on NTAS4D - Running
MSDTC on NTAS21 - Unknown

When I complete the wizard, I receive the message listed above.

The Servers are in two separate locations, each behind a NAT enabled router
using the internet to transfer data between the publisher and subscriber.

Any Suggestions will be greatly appreciated.


Thanks
Steve