What does Rohan mean?
From Sanskrit rohana (रोहण), 'rising, ascending, growing'. Some sources also give 'sandalwood'; the core attested sense is 'ascending/growth'. (Note: an unrelated Irish name Rohan/Rowan also exists.)
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.