Ah yes, you have discovered one of the many unfortunate shortcuts taken in this product.
So it appears that the CABI portlet is simply an HTML display window, it really has little or nothing to do with the actual CABI display.
What it does is redirect the UMP URL to the CABI URL and then display the results. So, once the window displays, all the rest of the interaction is directly with the CABI server and bypasses UMP completely.
In order to get it to work, the address of your CABI server needs to be externally resolvable too.
This gets tricky because in a simple configuration, your internal addresses are not going to be externally available and vice versa.
I ultimately gave up on getting it to work as I required it to - the big hurdle being that I didn't have the second public IP I could dedicated to the purpose of allowing external access. Plus the cost of arguing with my network security group the need.