Have you been watering it every day, or even twice a day? That's certainly what I would be doing in those kinds of temperatures.

That's the problem.

When a plant is stressed, it sacrifices top growth to save its roots.

Lantana is very robust; even after it seems like a lost cause, it may recover.

I once got a pitiful lantana at a plant swap, planted it, watered it faithfully even after it looked like the plant was deader than a hammer...

Late in the autumn, it sprouted new leaves, & the next summer it flourished.