Hello,

First of all you may not fulfill the education qualification criteria.To appear in JEE advanced 2020 a candidate must have passed 10+2 or its equivalent in 2018, 2019 or 2020 are eligible.

Suppose that you wish to appear in 2021 you must have completed your class 12 in 2019, 2020 or 2021 but you have passed class 12 earlier than 2019 so you will not be eligible to appear in JEE adavanced which makes you ineligible to get admission in IIST or IISER.