DIAGNOSIS Implicit Beamforming or Smart Connect is causing client compatibility issues, or the channel is congested.
FIX Log into routerlogin.net. Go to Advanced > Setup > Wireless Setup. Uncheck 'Enable Implicit Beamforming' for both 2.4GHz and 5GHz bands. Also, disable 'Smart Connect' if enabled—this will separate your 2.4GHz and 5GHz SSIDs. Apply settings. Reboot router and reconnect your device to the 5GHz network specifically.
IF THAT DIDN'T WORK Manually set your 5GHz channel and width. In the same Wireless Setup page, for 5GHz, set 'Channel' to 149, 153, 157, or 161. Set 'Channel Width' to 80 MHz. Apply. Also, ensure 'Enable 20/40 MHz Coexistence' for 2.4GHz is unchecked, as it can cause interference.
HEADS UP For the fastest speeds, your client device must also support 802.11ac and 80 MHz channel width. An old device can slow down the entire network if QoS is misconfigured.