I think there is some confusion about the AX1800's SoC. There is a router sold under the name 'Archer AX1800' https://www.tp-link.com/us/support/download/archer-ax1800/ which is different from the AX21. I have the AX1800 and I can confirm that it is not Broadcom:
DISTRIB_ID="OpenWrt"
DISTRIB_RELEASE="Attitude Adjustment"
DISTRIB_REVISION="unknown"
DISTRIB_CODENAME="attitude_adjustment"
DISTRIB_TARGET="model_mtk_mt762x/generic"
DISTRIB_DESCRIPTION="OpenWrt Attitude Adjustment 12.09-rc1"