it seems that the issue, is that some devices were getting both dynamic ip address and a static one

does the router knows not to assign an IP address to a MAC which we reserved an IP?

that did not solve it

Hello ​@orgerson , thanks for your message and welcome to our Community :)

A couple of days have past since your post, has anything changed? It's certainly strange that you only experience this problem on a phone and not with a laptop. Have you already tried to run the KPN Wifi manager? This is an online tool that helps you improving the wifi. 

I think the DNS servers are slow
I changed to google DNS servers I think that solved the issue

I will check the wifi manager later on.

Issue was solved by turning off IPv6 in the device

The device does not automatically revokes a previously given Ip address when you try to reserve it for static, it was unrelated

Thanks for sharing this with us ​@orgerson :)