Port Forwading on BT Smart Hub 2

I am having issues trying to set a port forwarding rule for my Plex Media Server on a NAS connected to BT SH2. When I open management page for router it shows a port forwading rule already set for my NAS ( WD Home Cloud) . This showing as 'greyed' out boxes set to use ports other than 32400 recommended by Plex. I am unable to delete or alter these settings and have tried factory rest on BT hub all without success. Any suggestions how to change these old settings welcome.

I am wondering if you have UPNP turned on and it has automatically created a rule?

I do have uPNP enabled . This is a requested setting by NAS supplier but even after turning off I still cannot delete the Port Forwarding rule I need to change.

Thanks I persevered and your suggestion was indeed correct. Many thanks problem now solved.

Great news.