Q.Planting Mandevilla in a Flower Garden
Last winter I brought my 2-year old Mandevilla inside the patio. Now through this year I transplanted it into a rope tub sized pot. Now that the roots are becoming increasingly in size, would it be possible for me to plant it before the fall and kept plenty of mulch to protect the roots? Would it survive a winter in Zone 6?

Your plant is beautiful.
Unfortunately your Mandevilla will not survive in temperatures below 50 degrees F.
Here is a link with more information.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm