Transmission shutter at 40 MPH while maintaining speed?

A shudder at a sustained speed in a CVT is usually a sign of a failing internal chain as it slips on the sheaves of the variator drive. However if you wish to do a fluid change it is a bit involved and you need the proper charge pipe KV311039S0. With that you raise the vehicle up level so you can get under it, then you need to remove the drain plug from the pain. Let it drain. Then you install the charge pipe in place of the overflow plug. Install the drain plug. Next you insert the hose into the pipe and add 3 liters of CVT fluid. Now remove the pipe and install the overflow plug. Now you start the engine and shift through each gear while holding the brake. Keep it in each gear about 5 seconds. Shut the engine off. Now you repeat the drain and fill process, except this time once it's drained you replace the gasket on the plug and reinstall it. Now you reinstall the charge pipe and add 3 liters of fluid. Now reinstall the overflow plug and start the engine and run it through the gears again. You want to get the transmission barely warm. Now remove the overflow plug and let the fluid drain until it stops. Replace the gasket and install the overflow plug. Clean up and you are done.
Your vehicle has the RE0F10E CVT in it and there are a few videos on that unit and the shudder they develop from the variator.

CVTs don't have any adjustments other than fluid changes. I'm actually surprised it has made it that far on the original unit. They commonly fail before they hit 100K (many under 80K) The charge pipe is a tube that screws into the overflow port on the underside of that transmission, not where the dipstick went. What most of the online videos tell you is the dump and measure method. Basically, drain out the fluid. Put the drain plug in, measure the amount you removed and then add back that amount. That will work, but you won't know if it's actually full.
This video shows the overflow being checked at around the 7.5-minute mark.
https://www.youtube.com/watch?v=x1bZiTNvT7k
That is where you would use the charge pipe. Normally you use the pipe so that you can fill the trans and check the level at the same time. However, you can fill through the dip stick hole if you wish.