Hi Amina, welcome! I'm sorry to hear you're experiencing difficulties signing up for a contract. It's possible with a Dutch residence permit to arrange a mobile subscription, just like you did for the internet services. My advise to you is to return to the store and try again. If they again say that it is not possible then they need to really explain why. The Dutch residence permit can't be the problem. If you have any questions, let me know 
I’ve got a similar issue, I’ve just relocated to Amsterdam and have my internet with KPN. I Want to get a SIM Only 5G contract for my mobile, however when I went into the KPN store on Saturday with my brand new residence permit I’d only just collected, they said it’s not possible because my permit isn’t valid for 12 months, even though I’d only just picked up the permit that morning. I’m on a one year contract to begin with, with an employers statement that it will be renewed at the end of 12 months. Is this sufficient to get a SIM-Only mobile contract with KPN?
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