Nothing happens on left to right because the StuG doesn't have turret rotation or gun traverse. It's HLs design, it's not proportional, just turning on a motor. The best way to over come it is to move to another system that allows a hobby grade radio and use servos for elevation and traverse. It'll require setting up a traverse pivot, which is a bit of an undertaking but doable.

Here's mine using a DBCR3 with a 6 channel Tx, using HLs elevation and servo traverse:

https://www.youtube.com/watch?v=CVa-qevzzdE