You need to access settings with the remote control thru the front screen, not the App…select screen off. The full description is on the Naim website for the Nd 555. If it’s flickering, this is a faulty unit and you should as well talk to Naim via the dealer.
I had an Atom which intermittently got a random greyscale pixellated display which was RMAd.
Unless there’s a known bug that causes this, I think you have to consider a display or display circuitry fault, which should be addressed on any device let alone an ND 555. Talk to your dealer.