Poor Customer Service

So I bought O2 sim on 03.09.2018 from Mall of Berlin Branch and the consultant named XXXX XXXXXXX gave me this package of 5 GB data with the promotion of 150 GB data with the Huawei Device. The total bill was 113,98!Euro. From Saturday onwards my internet wasnt working so Iogged in today and checked that my data of 5GB is consumed but he said he has given me 150 GB data. Today I went to this branch of Mall of Berlin again with my problem that my Data is over and it is supposed to be 150GB. So the lady over there said me that I can not help you this promotion is over and this was supposed to be activated by 3rd Sept which I had no information about and she said she cant help me because the other guy sold this promotion so she can not help me. I said I have invoice it will mentioned in it she said it is not on invoice and I can not help you for this..
So first of all why it is not on my invoice are they not recording it that how many people has bought this promotion and I told him this is my first time usinh O2 so I have no idea and even though he didnot activated that for me.
2ndly so if this person leaves the company will they close the branch because he had the conversation with me and the other person can solve my problem. It is very unprofessional that they do not help the customer at all. I have paid for 150 GB not 5 GB and when I asked her can you give me your manager email address so that I can escalate my issue to him she said you can write a letter and they will reply with in a week. So I dont understand that O2 being such a big company can not help their customers to solve their problem via email even in 2018?
O2 being such a big company is having such a poor customer service because when I am calling the helpline they are not answering and I wrote on my O2 portal I didnot get the reply and when I personally went there the staff was not friendly and not helpful at all!
I will never recommend O2 to anyone.

Edited by o2_Ubbine: Please don´t mention the full Name of our employees.