(in my case 192.168.56.1), whereas one would need 71.174.62.16

This means you try to get information that is stored in your router's and/or firewall's  (NAT, DHCP,... ) routing/mapping table.
I'd assume you need to use some utility of your OS using $ZF(-1,...) $ZF(-2,...) to get the outside view of the inside server

This might work inquiring nslookup on a DNS server outside your NAT, ...  Though I never tried it

Hi,
without any  REST (at that point)  as you expect to hand action from front-end to back-end:

If your front-end is able to listen on a dedicated IP port  then all you need is to send your data to the (negotiated?) IP:port.
Take a look to %Net.HttpRequest 
http://docs.intersystems.com/latest/csp/documatic/%25CSP.Documatic.cls?P...

From point of view of Caché your front-end acts then like a web-server receiving data.
Your reply from front-end  signals to Caché that the front-end is ready for the next junk / or terminates the stream

The fact that the table is external  is just a concern of storage strategy in the generated Caché Class
To your COS code  it's just another class projected as table.

Use it by your preferred ResultSet Class
It might be a good idea to check access first from Mgmt Portal to verify access rights.

for the example:
http://docs.intersystems.com/latest/csp/documatic/%25CSP.Documatic.cls?P...