@GeertCU
I did what you suggested. And I was surprised. They called me within 5 seconds that I had place the call.
The person from the callcenter told me that the iPhone itself does not block 4G for any providers. The iPhone doesn't need a carrierupdate either (at least that's what he told me). He said that the problem probably was that my abonnement with Proximus was not 4G compatible or that the 4G option was not enabled for my number at proximus.
When I told him that I have the correct abonnement and that the option has been activated he asked me if I could go through a few steps with him.
I told him that I had already tried almost everything.
He then suggested me to set up the iphone as a NEW iphone, without putting my backup back, as the backup may be corrupted and may be blocking the 4G from being enabled.
This is the only advice he could give me as extra as I explained him all the other things that I already have done.
So who wants to volunteer and setup there iphone as a new one ;-)
I'm afraid to try it out, to find out that it doesn't work and then having to put all my stuff back manually.