Keep in mind that you should use a controller to control portamento in real time. The knob is not going to give you acurate portamento. The knob on the interface is a way for you to track midi controller automation in your files, it is for reference only.
In order for portamento to work properly, you need to have the phrase slurred/legato by adding sustain pedal controller information to the track on midi CC 64.
Then on the notes you wish to have portamento you need to add data to midi CC 20 for the value of portamento you wish to add.