Hello Buddy! Hope you're doing good.

The eligibility criteria of getting 75% in 12th board is only for getting eligible to attempt the exam and it will not make any effect of losing NIT even after getting a rank below 5000 in jee mains or any other rank as your 12th marks will not be counted for jee mains or advance only jee exam score will be valid but you have to score min 75% as aggregate to take admission through your JEE score. This is commence from the year 2018. So no worry. For knowing your possibility of getting a seat, you canuse the college predictor tool given below :-

https://engineering.careers360.com/jee-main-college-predictor?utm_source=qna&utm_medium=jee_cp

Hope this will help you.

ALL THE BEST! WORK HARD !!