Plugins on the 7.1.4 buss

So I have an ADM file loaded into Nuendo. The audio is very dull, so I want to place an eq on the 7.1.4 bus (in the mix console) and add some eq.
I am placing the plug in ‘after’ the dolby renderer, but the plug in only seems to effect the Main 2 channels. Do I need to purchase specific surround plugins for this task?

No. The built-in plugins should do what you need.

Depends on which plugin you are currently using.
Most -if not all- Steinberg stock plugins should be working.
If you use, for example Waves, then indeed it will not work.