Dear Mr Sant,
there is no possibility to defer your admission to the winter semester. But if you cannot be in Hamburg on time to start your studies in the summer semester because of visa issues, there is a possibilty to start your studies late if the department of mathematics confirms that a late start is possible. So please contact the department and describe your situation to them: https://www.math.uni-hamburg.de/en/stud ... buero.html
If they confirm that you can start late, your enrollment for the summer semester will be valid although you are not yet in Hamburg to take up studies in April. In that case please send us the confirmation of the department via https://www.uni-hamburg.de/studium
Please note that you have to enroll by sending us your enrollment application within the deadline which is noted in your admission letter and that you have to pay the semester contribution for the summer semester for your enrollment to be valid. You will not be able send proof of health insurance within the deadline, since you can get that proof only if you are in Germany. This is no problem for your enrollment - you can hand in that proof when you have arrived here.
Another possibility for you would be not to enroll now and to reapply for the winter semester. so please ask the department of mathematics for an estimate of your chances to be admitted again - if readmission is very probable, you might save money by not enrolling and paying the semester contribution now.
You also sent your request via our contact form - please consider that request answered with this reply. And please refrain from sending further requests via different contact channels - since those channels are tended to by the same team you won't get quicker answers by sending multiple requests.