Q.How To Overwinter Mandevillea?
Zone 7A, have an unheated garage and cool basement. Several are in pots, some in the ground and can be dug up.
The easiest way to overwinter your Mandevilla plant is to (1) originally plant it in a suitable container, move the container to a patch of full sun and then move the entire plant indoors when temperatures approach about 50°F to allow Mandevilla to go dormant in a dark, cool space away from hot/cold vents. Do not allow the soil to get bone dry. Before bringing the plant inside, check for pests that you might be also bringing inside. Expect leaf loss during this time. The garage is usually fine unless there are extreme winter weather events. Plants that are growing in the ground outside need to be transplanted into a container.