Abonnement and new mobile phone. I didn't know this was possible

Hello!

I really need for help.

8 April I’m opened account but I don’t know that I can buy a new mobile phone and include in it my subscription for unlimited data. I just come to Netherlands and don’t know this. Now I saw an ad and want to buy iPhone 13 and keep my subscription.   An employee told me that it’s impossible. I can’t change or reject my subscription and must only open a new one. What I can to do? This is not entirely correct in my opinion.  I want to keep my contract and add an option to it (mobile phone abonnement) not a single month has passed since the subscription, so this is possible I think. Help me or I want to terminate the contract for improper provision of services.