I'll answer this regardless of the poential duplicate of the 'Can you help me with my software licensing question' because it does specify a very particular scenario which I have had some experience in.
Theoretically, you could go with either external connector (EC) or SPLA, but their user cases and cost are very different.
An EC is designed to allow external access to essentially your intranet, so if your business is not IT and you have a bunch of people working for your company who are using MetaTrader running on a Windows server, then you would purchase an EC to make your internal service available to outside users.
On the otherhand, if your service is not primarily intended for internal users, and you are an IT company and on the Microsoft Partner program, then you can provide a service to external users on the basis of an SPLA (Service Provider License Agreement). I recall that one of the conditions of the SPLA is that no more than 50% use for the software will be internal, although this may vary by year of signing and country.
In your user case I'm guessing that the SPLA will be a more flexible and cost effective arrangement. Taking this route however, you will not be able to use your Windows Standard OEM license.
Another potential scenario is to add terminal services CALs to your Standard OEM license (if possible) and set up a VPN for each client to access them - but this is probably not supported legally, after all, your clients are clients and not remote workers, right?
