DIAGNOSIS You're likely not connecting to the 5GHz band, or you're connected to it but with a weak signal or on a congested channel. The 2.4GHz band is slower and more crowded.
FIX First, check what band you're on. Log into the router and go to Wireless -> General. Make sure your 2.4GHz and 5GHz networks have different names (SSIDs). Connect your device explicitly to the 5GHz one. Then, in the 5GHz professional tab, set "Bandwidth" to "160 MHz" if your devices support it, and set the "Control Channel" manually to a clear one like 36 or 149. Run a speed test close to the router.
IF THAT DIDN'T WORK A feature called "Smart Connect" can sometimes steer devices to the wrong band. Turn it off in the Wireless -> General tab. Also, check for QoS (Quality of Service) settings under Adaptive QoS. If it's enabled and misconfigured, it can throttle speeds. Try disabling it temporarily as a test.
HEADS UP The top Wi-Fi speeds (like 4800 Mbps listed on the box) are a theoretical maximum under perfect lab conditions. Real-world speeds are always lower due to walls, distance, and device capabilities. If you're getting solid 500-800 Mbps on a 1 Gig plan over Wi-Fi, that's actually performing well.