That would be Arris and more than likely your ISP if this is ISP supplied since they control the updates.
https://en.wikipedia.org/wiki/IEEE_802.11ax also tells us it's not just firmware.
https://www.arris.com/products/nvg44x-xdsl-voice-gateway/ tells us this is ISP supplied and shows no sign of 802.11ax.
But all is not lost. https://en.wikipedia.org/wiki/IEEE_802.11ax#Access_points shows you could add 802.11ax with an Access Point.
Has the firmware for Arris NVG443B been upgraded to provide 802.11ax?