I am not sure exactly what you are trying to achieve with the endpoint configuration, especially with the phones not being local.
The endpoint configuration is not necessary to get your phones working with Elastix. The main purpose is to make it easy to provision phones on the network.
Depending on the phones that you are using, is whether you will achieve some moderate success.
One of the prerequisites for the phone provisioning to work is the ability on the phone to set the TFTP Server on the phone. For example the Linksys SPA range of phones, will only obtain the TFTP setting when they receive the Option 66 as part of the DHCP request.
To provide a better idea of the concept, this is generally the simplified flow of what is occuring.
1) The Endpoint configuration scans the network looking for Phones. These phones then report their MAC address.
2) The endpoint configuration then creates a file based on the MAC address of the phone and places it in the /TFTPBOOT directory.
3) When the phone is rebooted, providing it has either the ability to set a TFTPBOOT IP location, or obtain it via DHCP, it will access the TFTPBOOT directory and look for a file that matches its MAC address
4) It applies these settings to the phone and hey presto you have the phone online
Now the endpoint configuration tool is more suited to phones on the local network.
However, this does not stop you making your own config file for your phone, naming it with the correct MAC address, and placing it in the TFTP Directory.
The main issues that you would come across is opening the ports on your router for TFTP, setting the TFTP address etc.
Personally, I would not bother trying to use Endpoint provisioning, just hand code the settings on the phones.