"Unfortunately" ECP is the only method to connect remote databases. Maybe it's a license-server-problem. Could you please check that both instances are using the same license server?

Let's say, both systems are placed in a domain jetsons.com. There are two systems each of them has a Caché-instance running. System-1 is called betty and system-2 called wilma. 

So we have:
System1: betty.jetsons.com / Caché-Instance = barney
System2: wilma.jetsons.com / Caché-Instance = fred

System1: License-Port = 4001 (you may use another port instead of default), License-Server = betty.jetsons.com
System2: Same licensing-information as on System1 (betty.jetsons.com:4001 (or another port you defined there)

After doing so, restart both instances and check cconsole.log if license server was started on both systems.

And if that doesn't help then you should contact WRC.

Please keep in mind that the service ECP (Administration -> Security -> Services) must be running.

Maybe you need an upgrade of the license. I remember that we had to do it when we installed a newer version. To do so you also have to contact Intersystems-WRC

Hope, that will help you :-)