There is a problem in your cvt valvebody. Either oil pressure sensor issue or valvebody is unable to make specific oil pressure. It gives shudder at speeds of 40-60km/h only, but not lower or above then that.
If you can google it then you will know that in USA they will give you a warranty claim for this. Take it to dealership and claim new gear under warranty.
Just to let you know new gear price is 10 lakhs now and used is worth 2 lakhs. If it goes with calibration then you're good to go.
If you're in Lahore then go to Toyota Airport and meet supervisor Basit bhai. He's a very knowledgeable person and if there is a problem in your car he will confirm it for you.