What types of moisturizers help dermatitis?

Protect. Avoid things that make you break out, soaps & wetness. Wash your hands only when necessary. Wear gloves when needed. Wear clothes made of cotton. Bathe only with a small amount of mild unscented soap, such as dove. Keep the water temperature cool or warm, not hot. Use the medicine your doctor gave you. Use a plain moisturizer daily. Avoid scratching or rubbing the itchy area. Manage stress.
They don't. "dermatitis" just means "inflammation of the skin". In other words, pretty much every rash is a "dermatitis". Without a more specific idea of what is going on, it is impossible to recommend treatment. Moisturizers decrease skin dryness (sometimes), but they have no anti-inflammatory ingredients, and thus are not useful for treating most kinds of dermatitis.