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What zone are you growing in?

Mandevilla are hardy in zones 9 - 11.

The plants will not survive in a colder zone and should be brought indoors when temperatures drop below 50 degrees F.

In warmer climates the plant will generally can be trimmed back in the winter about 6 weeks before the last frost. This will help produce a nice bushy plant.
Trimming of the vine all summer can help keep it in control.

Every 2 to 3 years you can trim the plant back to the ground to reinvigorate the plant.

If you are in zone 8 or colder, your plant may not have survived.
If you are in zone 9 or warmer, you plant should began to grow when the weather warms.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

If the roots are protruding from the container, it is likely time to repot.

Choose a container that is slightly larger and wider then the existing pot.

Mature pots can generally use a 12 to 20 inch pot. You generally move up 2 inches in pot size as the plant grows.

Make sure it has adequate drainage and use a light fast draining potting mix.
Pre mixed potting soil that is made for cactus is a good choice.
Do not trim any roots or tubers.

The plant should have about an inch or so of new space all around the root ball.

Do not plant in too large a container, as this can lead to over watering and root rot.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

The heat and sunlight could definitely be affecting it, but typically yellowing leaves does mean the plant got too much water. Mandevillas need sandy, well-draining soil, support and some shade. Cut back on the water, cut off any dead leaves and double-check your sunlight situation.

This article may help you:

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

The links below will help you pinpoint the cause of yellowing leaves.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

Hi there. Aphids arrive in groups and invade very quickly. I buy an organic aphid killer spray. and go around every Sunday to check and, if needed, spray. The Same with slugs and snails I don't wait until they are taking over before doing something.
I learned from an Australian gardening radio program called "The Garden Clinic" to be PROactive in the garden rather than being REactive

If needed put the spray ready right beside the plant....

Good luck there

Humus in Brussels

Here is an excellent link with care requirements.

https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm