Diploma holders have lots of opportunities f
For higher study, you can opt for engineering entrance examinations . You can give B.Tech Lateral Entry Scheme , AIME Certification, Graduation in Study Domain,
government job then start preparing for the competitive exams like RRB JE, RRB ALP, state-level Junior Engineer examination, bank exam preparation, etc.

For more clarification after diploma you can visit here
https://engineering.careers360.com/articles/b-tech-after-polytechnic/amp
https://www.careers360.com/careers/career-after-polytechnic

Thank you. I hope this answer helps you.