Just wanted to add that even if Mandevilla drops all leaves it is not dead. I stripped mine of all leaves due to persistent mealies hiding near the stem. And pruned to old thick brown stems. Wiped with 1:1 alcohol ev week for a month. and kept it in the sun warm in SO FL in winter (was around 70-75F during the day) and it regrew in a few months. But it was slow at first. So do not chuck it , keep it warm and sunny /under lighst and it will survive.
What you see on my pic above is growth from Mar to Aug this year from bare old stems.
Q