From 2.5T to 3.5T you will need an in cab controller to work your trailer brakes. There are two options, first a hard wired system that is pre wired to the tow vehicle. Second a wireless Credo system that can be used in any capable vehicle. See this video for more detail and what might be best for your situation.

Do you have a Credo wireless braking system? See this video to understand how it works. See written instructions here.