That's odd, from reading the instructions one would have thought that control would be interchangeable.
I use a Humax Youview box and Panasonic TV - the Youview volume output is set to 100% so TV volume is controlled from the panny remote. However, I could if I wanted to quite easily reverse that control - set the panny TV volume to 100% and use the Youview remote for controlling volume (I don't because I find the the former method smoother).
Following those linked instructions I would program the TV code, set the TV volume control to a reasonable level then select DISH remote volume control in System Setup and save. To my mind this should set the box volume control as default - 100% being the level to which you previously set the TV volume to.
To return to TV volume control, select TV remote volume control in System Setup and save. I see no reason why a code for a different brand of TV need be required.
To get Brexit through Teresa May fell on her sword - and missed.