If the X20 is like TP-Link's 802.11ac mesh systems, you may need to add a test device to the Deco app's QoS list to see the fastest possible speed. By default, most mesh systems are configured to manage multiple clients concurrently with decent coverage and speed, rather than optimising for peak speed on an individual device. You can add multiple devices to the QoS table, bear in mind this will be at the expense of other devices not on the list.
Unless you really are missing out with "only" 550 Mbps to a phone, then possibly better to let the X20 do what it is designed to do.
If adding the iPhone to the QoS list doesn't improve speeds, then it suggests a wifi environment issue (eg brick or stone walls, interfering wifi signals or other related problems).
