DIAGNOSIS Your device is probably stuck on the slower 2.4GHz band, or a setting is limiting throughput. Distance and physical obstacles are the usual suspects for band steering not working right.
FIX First, test close to the router. Stand right next to it with your phone and run a speed test. If it's fast there, it's a range issue. If it's still slow, log into the admin page. 1) Go to Adaptive QoS > Bandwidth Monitor. Make sure QoS is disabled for testing. 2) Go to Wireless > Professional. For both 2.4GHz and 5GHz, ensure 'Airtime Fairness' and 'Universal Beamforming' are enabled. 3) In Wireless > General, make sure your 5GHz and 2.4GHz networks have different names (SSIDs). This lets you manually choose the 5GHz network on your phone.
IF THAT DIDN'T WORK Check the Ethernet cable between your modem and router. If it's an old Cat5 cable, it can bottleneck you. You need at least a Cat5e or Cat6 cable to handle gigabit speeds. Also, ensure the cable is plugged into one of the router's blue LAN ports when testing a wired computer's speed for comparison.
HEADS UP Many phones show a misleading connection speed. The 'link rate' in your phone's WiFi settings is not your internet speed—it's the maximum theoretical speed to the router. Real-world throughput is always lower, especially with multiple devices active.