What does Ishaan mean?
From Sanskrit ishana (ईशान), 'ruler, master', also a name of the sun and an epithet of Shiva (lord of the northeast direction). Glossed both as 'the sun' and 'lord/ruler'.
What does Reyansh mean?
A modern coinage usually read as 'ray of light', often analysed as 'ray' plus the Sanskrit amsha (अंश, 'portion/part'), giving a devotional reading 'a part of Vishnu'. The Vishnu link is an interpretation built on amsha rather than a documented classical meaning.