Hello. To be eligible to apply for admission in IIST Trivandrum you must pass JEE Advanced 2019 with atleast 18% of total marks (360) i.e. 65/360 and atleast 5 marks out of 120 in each section for OBC-NCL. Though your percentile and rank may differ after the April session of JEE Mains but according to your current percentile your probable category rank is 24122 - 28122. With this rank you might be able to qualify for JEE Advanced.

Hope this helps. For more details like previous years cutoff etc of IIST Trivandrum check the below link.

https://www.google.com/amp/s/engineering.careers360.com/articles/iist-btech-admission/amp

Good luck!