Hi Ian_K,
Let me help you with this.
The reason why you are having problems sending CEC commands through DP is that DP natively does not support this feature and I would assume that is why you are using the DP to HDMI adapter.
However, it is important to check the adapter. Passive adapters are very; let’s call it, very simple therefore you can try with an active adapter. We do not recommend the usage of adapters because can interfere in the data transfer and video signal and we cannot guarantee the correct functionality of the device but you can give it a try.
Kevin M