The 14 days the tenant has to pay, did they start yesterday, Monday (the day after I physically delivered the demand on Sunday)? And what do I do when he neither receives the letters nor responds?

According to § 182, subsection 2, you can only terminate the lease agreement due to late payment if the tenant has not paid the arrears no later than 14 days after a written demand has reached the tenant. The crucial legal requirement is that the demand has reached the tenant — not that the tenant has acknowledged receipt, signed anything, or actually opened and read it.

Your second delivery — the physical delivery in the mailbox on Sunday, documented with photo/video, and followed up by an SMS the same day — is therefore what counts. It is not a requirement that the tenant has signed a receipt; it is enough that you can prove that the letter has actually been brought to the tenant's mailbox.

The earlier uncollected registered mail is not decisive now that you have a later, documented delivery.

I am not sure about the precise day-to-day calculation of when the 14 days exactly expire (whether Sunday itself counts as day 1, or whether the deadline runs from the following day) — I would recommend that you double-check with an advisor or your landlord association, so you do not act a day too early. But the basis for the demand — that it has reached the tenant, regardless of lack of acknowledgment — is in place.