You can try trains actually. The journey that takes 9 hours by a bus will be happening in 5 hours by train. And while there is one listed bus service there are several others that are there when you arrive at the bus stand.
There are however 11 trains that go from Guwahati to Dimapur. Any train that goes to Dibrugarh crosses Dimapur. Makes more sense as the journey will be way more comfortable and in lesser time.