iPhone 5s Dock Station and Volume Control

Hello - I have a iPhone 5s dock. The dock works with charging the iPhone as well as the line out. The problem I am facing is when I dock the iPhone, the Volume control on the iphone disappears. So when I am playing music, the volume cannot go up/down. I've tried adjusting the volume using the side volume buttons, but that does not work. The speakers that are connected to the Line Out, do not have a control either. Is there anyway to control the volume by using the iPhone?

Thanks in advance.

iPhone 5s
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