Praise be to Allah.
Cutting nails and shaving the pubic hair are not things that are obligatory for the fasting person in particular, but they are not things that go against fasting either. Rather the fasting person refrains from eating, drinking and having intercourse, which are things that invalidate the fast.
A fasting person should also keep away from sins and bad things, such as backbiting and spreading slander, which reduce the reward for fasting.
With regard to cutting the nails and shaving the pubic hair, these are matters that pertain to the fitrah (natural state of man), which the Lawgiver said should not be left for more than forty days. They have nothing to do with the soundness of one’s fast.
And Allah knows best.