Hello aspirant

Hope you are doing great there . IISER gives admission only on the basis of these 3 channels :-

Kishore Vaigyanik Protsahan Yojana (KVPY) channel

Joint Entrance Examination (JEE-Advanced) of the Indian Institutes of Technology

State and Central Boards Channel (SCB)

You can get a seat in IISER only and only if you get qualified in one of the three channels mentioned above

I hope this answer helps

Wish you all the very best!