Contract did not mention starting in 4 months automatically

Hi, 

I have signed up for a sim-card subscription and requested to keep my number (13th of August, I believe, 2021).

I received a sim card that with a note that said that I cannot use it until my contract starts. And that I will receive my contract soon. 

I received a contract indeed. And then I received a message on my phone that I cannot change abonnements because my old contract isn’t over ( with a different service provider) and I  have to wait for it to end. 

So I assumed that the number exchange and my subscription cannot go through. There was no communication about my subscription started for 5 months. Until in January I receive a random invoice (Kenmerk: 2223/1.20942296) for 18 euros that is said to be take on the 26th of Jan automatically. I cannot use the “factuur uitleggen” service , I get (Je hebt geen rechten om deze pagina te bekijken.
Probeer een van onderstaande opties om verder te gaan.) . 

In conclusion, I do not understand what the 18 euros invoice is for. Does that mean you are starting my subscription 5 months later after I ordered it with the other number? I did not see anything about this in the contract. Also, I am aware that I could only cancel the contract within 2 weeks after order. But I did not get any indication that it will go through with the other number even after decline from my previous provider. 

Please, help because that is a lot of money paying for two subscriptions for like a year. 

Aziya