can i get into IIST through JEE mains only if i am a st student or is jee advanced the only way to get in?

The Indian Institute of Space Science and Technologyoffers admission only on the basis of Jee Advanced. It doesn't consider scores of Jee Main. In order to get into IIST, you need to fetch good rank in Jee Advanced. IIST will release the application on September 27 for admission to B.Tech programmes 2020 through Jee Advanced and the last date to fill the application is October 7. IIST will release the rank list based on the applications. You can read our article http://engineering.careers360.com/articles/iist-btech-admission
The Minimum criteria to be eligible for Indian Institute of Space Science and Technology is that qualifying Jee Main with at least 20% marks in aggregate and 5% marks individually in all three subjects i.e. Physics, Chemistry and Mathematics. This is for general category candidates. For OBC NCL candidates, they must score at least 18% in aggregate and 4.5% individually in all three subjects and for SC, ST and PwD Candidates, it is 10% in aggregate and 2.5% individually in PCM.
In 2018, the cutoff percentage for General category was,
Opening percentage 43.89 while the closing percentage was 34.17 for general category in B.Tech Aerospace Engineering.
In 2017, the opening percentage was 64.21 and the closing percentage was 53.83.
This is how the cutoffs change every year as it depends on various factors. And this is according to last few years' data and this year the cutoffs might change.
You can also check other years' cutoffs here http://engineering.careers360.com/articles/iist-btech-cutoff