My underlayer is completely straight. It won't even bend with a curling iron.
One thing you can do is have your stylist cut the underlayer shorter so that it isn't longer than the rest of your hair that bounces up.
There's a chance that gel or mouse or another product with hold may help your underlayer wave, so you could always give that a shot.
Best of luck. Having two hair types is a pain!
Wave Pattern: 2A
Hair Properties: Normal/Low Porosity, Medium Density, Medium Texture w/Fine Straight Underlayer
Co-wash: Suave Naturals, Cure Care
Low-poo: Renpure Brazilian Keratin
Rinse Out & Leave In: Tresemme Naturals
Gel: LA Looks Mega Mega Hold Gel, Garnier Fructis Spray Gel