DIAGNOSIS Your device is connected to the 2.4GHz band, not the 5GHz band. The 2.4GHz band is slower and more congested. This is a band steering failure.
FIX On your device, forget the current WiFi network. On the router, disable 'Smart Connect'. Log into routerlogin.net, go to Advanced > Advanced Setup > Wireless Settings. Uncheck the Smart Connect box. Create two separate network names (SSIDs), one for 2.4GHz and one for 5GHz. Connect your speed-testing device explicitly to the new 5GHz network.
IF THAT DIDN'T WORK Check for QoS (Quality of Service) being enabled. In the web interface, go to Advanced > Setup > QoS Setup. If 'Turn Internet Access QoS On' is checked, turn it OFF. QoS on this model can throttle overall throughput if misconfigured.
HEADS UP For maximum speed, you must use a WiFi 6 (802.11ax) capable device. An older WiFi 5 (802.11ac) device will not achieve the router's top theoretical speeds. The speed test also requires a wired Ethernet connection from your modem to the router's yellow port.