Will I get a seat at NERIST (2021) as I secured the 7th position in the waiting list of Assam?

From your question, it’s not quite clear which examination you took. NERIST conducts 3 examinations, i.e. NEE1, NEE2, and NEE3. You can appear for NEE1 after HSLC. NEE2 is for after the 12th. NEE3 is for after graduation. 

If you have taken NEE1 or NEE2, there are good chances that you will be selected. Most students do their 10+2 and don’t join. For NEE2, students leave their seats, if they receive a better college. 

However, for diploma students, it is one of the best colleges available in the Northeast. So, if you are a diploma student then it will be quite difficult to get admission with your rank.