B.Tech

First of all, congratulations to get good marks in your 12th. Taking admission in NIT or IIIT is dream of many students in India. To get admission in NIT or IIIT, you need to appear for JEE Mains exam conducted by NTA every year. Every year 7-8lakh students appear to give this exam
There are 31 NIT and 25 iiit in india in which there are approximately 25,000 Sets for B.Tech in all NIT and approximately 5000 seats for B.Tech in all iiit. As you got 89 percentile in JEE Mains score, to get admission in NIT or IIIT bassd on your this score, it depends on that from which category you belong like general, OBC, SC, ST, PwD
If you are from general category, then you will not get any NIT and iiit
But if you are from SC or ST or PwD category, then you will surely get any NIT or iiit with best B.Tech branch as per your choice.

As you have completed Diploma, you can also pursue B.Tech in regular mode rather than distance mode from best India's college like IIT government University central university private universities, as this type of degree has more value and also more chances to get job than distance type
IGNOU conducts admissions for B.Tech distance in two cycles January and July. Check website to stay updated about the July admission details for B.Tech course from IGNOU. You can apply for the course till the last date as based on IGNOU guidelines. Students can apply for various Bachelors, Masters, Diploma, postgraduate Diploma, and Certificate courses both programme and honours courses
The course fee of the whole B.Tech. Course at IGNOU is INR 20, 000. However, the fees may vary from specialisation to specialisation. IGNOU offers distance education at an affordable rate for the students who face financial instability and still wish to study from a recognised university. The course is offered in Construction Management and within that the student has to complete 2 modules in the course. After the completion of those 2 modules, the student will be eligible for the advanced construction management. The duration of general B.Tech is 4 years. However, the maximum duration to complete B.Tech. At IGNOU is 10 years. The specialisation that IGNOU offers in B.Tech is Construction Management which is a course that is made for working individuals. Hence, to let the working individuals have their time and a flexible routine, the university offers 10 years for the completion of the course. The course has 8 semesters with practical exposure as well.