For reference, Plugable includes in the box with the UD-3900 a three foot long USB 3.0 cable in order to connect the dock to the host computer. The included cable is a 'hybrid' cable, in that it provides both USB-A and USB-C connectors. 

If the included three foot long USB 3.0 cable is not of sufficient length, longer cables up to six feet in overall length can be purchased separately. We don’t recommend cables longer than six feet in overall length due to potential performance issues. 

Examples of alternative cables that our customer's have reported success in using are:

Six foot long USB 3.0 Type-A to Type-B cable --> https://www.amazon.com/dp/B00C7RXR72

Six foot long USB 3.0 Type-A to Type-C cable --> https://amazon.com/dp/B07W81GXV9 

NOTE - Plugable cannot guarantee the compatibility or performance of any 3rd-party USB cable.