PS4 Manual IP / DMZ

Hi

After experiencing some online problems on my PS4 (kicked out of parties, trouble connecting to players) I decided to put my PS4 into a DMZ. At first, I noticed my PS4 would always use the same IP address whenever I started the console, even though it was on automatic IP address. So, I just put that IP into the DMZ in my router and everything was fine.

Now, it decides to give 1 of 3 different IP address when I start up. So I decided to manually set up the internet connection and give the PS4 a manual IP address which I done, and filled in all the DNS, Subnet etc... and put that manual IP address in the DMZ instead. I restarted everything and now ran the internet connection test... Obtain IP? Check. Internet Connection? Check. PSN? Check. Then it gets to NAT Type and waits for a while and then says NAT Type failed?

How can I solve this please?

Dan