I set up this Deco today but I'm getting very limited speed on one device- iPhone 17 Pro Max.

All other devices get the full speed of my ISP- this includes other iPhones, iPads, Mac and Samusung phones. 
 

My internet speed is 500Mbls down, 50 Mbps up

On my 17 Pro Max only- the speed test yields ~12 to 23Mbps down.

What can the issue be?