[ADVICE NEEDED] Unresponsive HR after request for delaying start date

I got an offer for ECM/Equity Sales rotational internship at a well-known Asian bank's New York office. They just started formal internship recruitment so I'm the first batch. I met the CEO at a Career Forum and got the internship because he really liked me. I met 4~5 of the senior members for Superday and toured the office twice, and I also went out privately with some of the senior members of the company to grab coffee/lunch after accepting my offer. I'm supposed to start around late May because there's a very important investor meeting in New York around late May and they wanted my help. However, I learned that there might be an OPT processing delay this year (I'll probably receive OPT around mid-June) and I contacted the HR about the situation. Having minimal experience with hiring international students, he thought that I messed up the OPT application and wasn't able to obtain work authorization so he was very unhappy. I explained in E-mail that it is a common issue amongst international students and I wanted to meet with him in person to discuss possibly delaying my start-date. He just stopped responding to all my E-mails (I followed up once). I'm a little worried about what I could do moving forward and is this a common situation? Is it appropriate if I just show up in the office if I receive no response again? Thank you so much for your time and help!