It is the customer's responsibility to ensure that the APL and TAE connector are accessible at the technician's appointment. The presence of a specific person is not necessary. Any person of full age can let the technician in. If the APL is in a locked room, the customer must unlock it (or have it unlocked) or obtain the keys. The technician does not, of course, have any keys to the customer's house.

If the customer does not comply with his duties, he can be charged for the additional effort, e.g. for a further journey of the technician. A further technician appointment can be arranged via the hotline. 
See also:

https://hilfe.o2online.de/english-o2-community-broadband-dsl-70/dsl-welcome-letter-a-short-explanation-526842

https://www.o2online.de/service/dsl-aktivierung/

Hello. Thanks for the reply. But I tried many times contacting the customer care on the hotline number but I am not able to talk to the customer care executive. I am not sure as to how to proceed as I am still learning German and not fluent with complex sentences. Please can you guide me.