Check these websites for help. 
http://extension.msstate.edu/news/southern-gardening/2022/mandevilla-and-dipladenia-although-similar-flowers-one-vine-other-shrub
https://www.pennington.com/all-products/fertilizer/resources/why-plant-leaves-turn-yellow-and-how-to-fix-them#:~:text=to%20nutrient%20deficiencies.-,Poor%20drainage%20or%20improper%20watering,drought%2C%20has%20a%20similar%20effect.
https://extension.illinois.edu/blogs/extensions-greatest-hits/2007-08-28-yellow-leaves-can-indicate-plant-problems

What about the brown spots on the leaves. Can Dipladenia get blight?

Thanks.

I do not know. I've never heard of this plant before your question and did research on it. These questions are answered by horticultural volunteers. I will try and get it reassigned to someone from the UMN staff.  

Thank you for writing.
There are several possibilities.
The simplest would be sunscald from being exposed to bright sunlight for more than 6 hours per day. This would be concentrated on the south side of the plant.  A picture of the whole plant with a directional clue would be helpful.
This plant is susceptible to a variety of infections including anthracnose and Southern Wilt but I need a picture of the whole plant to go much further. https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm#:~:text=Sometimes%20the%20reasons%20for%20yellow,with%20Southern%20wilt%20eventually%20die.
Finally, it could be insects, here I would need a close up of the underside of a leaf.