Your tariff is o2 my Home L? Did you check your connection speed with your device connected via LAN cable with https://breitbandmessung.de/ ? 

Hi, my tariff is o2 my Home M and I’ve already checked the speed couple of times but it seems there is a certain problem in the connection. 

The contractually agreed speed for the o2 my Home M tariff is 5 to 50 Mbit/s for download. See product information sheet.

Does your Internet username end with @s93.bbi-o2.de or @s92.bbi-o2.de?
What is the technical maximum available speed shown in the o2 DSL availability check for your connection address?

Sorry, it is indeed up to 50 Mbit. 
My username end with @s93.bbi-o2.de and I have checked the capability of maximum speed which is up to 250 Mbit. 

This is a VDSL connection and 15 Mbit/s is too low for that. If you are sure your VDSL-Routers supports VDSL2 Vectoring and is not the cause for that:

Try the o2 DSL Hilfe App. If it refers you to the hotline, please call there. If you do not speak German, it is best to get help from a German-speaking person.

Please note that there is no support for problems that are caused by a router that is not provided by o2. Unfortunately  you did not mention the type of your router and how you set it up. 

I am using a router provided by o2 and it is set up by a person coming from o2.

Really? If wasn’t aware that o2 offers a set-up service for routers because all 5 different router modells which o2 offers for rental are designed to be set-up by the user himself. 

How did you order that service, how much does it cost?

I visited a o2 shop on Nordstraße and explained that I would like to have mobile phone plans as well as an internet service for my house. 
 

Couple of days later, a person from o2 came to our house and made the all connection (both in the basement and in our home). We did not pay anything for this service. 
 

However, we currently have very low internet speed  which causes problem especially in these “corona days”. 

Hi again,
I could not reach any “real” Person in the Hotline since it is all German and I dont speak German. I’ve got a serious Internet Connection and slow Speed Problem here but I cannot reach anyone who can solve my Problem.

This did not work! Although, I was connected to a technican, he was not able to speak English. 

I think this is not quite right: It doesn't work because you are not able to speak German. Therefore, I have already pointed out to you twice that you should get help from a German-speaking person if you do not speak German yourself.

Translation services are not part of the scope of services agreed with o2 and the contract language and common language in Germany is German.