wifi speed limited

Hi! Ia have AC86U with Merlin FW 384.17 I noticed a limited speed at 2.4 GHz. No device connected can not exceed 144Mbps - Wifi Log show 144/144 and signal -34 dBm (distance 2m). I'm in the country house with no other WiFi signal (no other routers near or other disturbances). I know, that I...