Job offer timeline

I'm a dietician with 2 years of experience, I have cleared an interview recently in one of the top FMCG. They had discussed even the salary structure and was informed that offer letter would be sent within few days before September. But I haven't received yet.

How can I reach out to them now as I have already mailed them multiple times but they gave assurance everytime.

Any suggestions on how to reach out to them ??