If you receive your visa late, a late arrival is also possible. There is no deadline when you have to be in Ilmenau at the latest. Arrival is possible during the whole semester. The prerequisite is that you have enrolled in time for the respective deadline.
You can download a certificate that you need to be present in Ilmenau from the application portal ("Useful documents").
However, if it is foreseeable for you that you will not make it to Ilmenau by mid-May, then we recommend that you do not enroll and start your studies, otherwise you will have to catch up on too much content.
If you choose to enroll, it is possible that online lectures will be held in your study program. Unfortunately, we can't tell you exactly which subjects this applies to. This is due to the fact that the lecturers are free to decide how they structure and hold their lectures.
But we can tell you from experience that materials are available online for almost every lecture. These are offered via the online platform Moodle. With these materials, you can usually catch up on what you missed in a lecture. For this, however, you must enroll in time in any case. This also works from your home country More information here.
Also, check your university email box regularly, which will be available to you after successful enrollment. Here you will receive important information about your study program by mail.
You can find all further information about your arrival on the we4you arrival website.