Exact cutoffs can't be specified as cutoffs keep changing from year to year, institute to institute depending upon various factors like number of candidates appearing and also the paper difficult level, etc etc. But yes If I were about to give you a mere rough idea then try to get 90 percentile and above in JEE mains to secure electronics and telecommunications branch in the top 5 NITs.

Some rank wise cutoff for NITs EXTC branch are given in the link below...

https://engineering.careers360.com/articles/nits-cutoff-for-btech-electronics-communication-engineering

Best wishes. Thank you.

While it depends on your state of domicile. Most of the NITs keep cutoffs lower for a student coming from the same state. But for a safer side 94-95% percentile can get you a decent NIT. BEST OF LUCK