When you signed a contract in a shop you can't cancel it between 14 days. It's possible for contracts by telephone or online only . 

You can cancel it to the end of the 24 month period. 

Only the shop can help you. 

Hello @Nat_Ska, 

first of all welcome to our community

Bollermann has already given you the right information, unfortunately there is no right of withdrawal in this case. 

I would suggest that you contact the shop where you signed the contract again. Or perhaps you have already done so?

Best regards
Cora

 i contacted the shop they told me it isn’t possible anymore. My only concern actually is that I spent quite a lot of time outside Berlin for 1-2 months every time and was wondering about the roaming costs. The shop told me that there are no extra charges for Europe and everything (calls, sms etc) are the same as in Germany. Is that correct? My contract is the O2 mobile unlimited smart online. Also, they told me if I leave permanently Germany I can Abmeldung from the contract. Also, the contract connectivity is better than the prepaid ones. 
@Bollermann thank you for your help

@o2_Cora 

Hello again @Nat_Ska 

"no extra charges for Europe and everything (calls, sms etc) are the same as in Germany. Is that correct?..." 

NO

In EU Roaming is Unlimited not Unlimited.

You have round 44GB data,. calls and sms are "free". 

@Bollermann yes I am aware that internet is 44gb, which is ok. But what do you mean by ‘free’ calls & sms? Not sure about the quotes here, are they actually unlimited - free of charge? 
thank you a lot for your reply! 

Calls and sms in/from EU  are free with flat. 

Calls and sms from germany to EU or other are not free. 

Hey @Nat_Ska,

I see that Bollermann has already given you great support. Do you have any other unanswered questions?

Kind regards, Lea