DD-WRT has a license agreement and NDA in place with Broadcom that allow usage of better, proprietary, closed source wireless drivers (binary blobs) which they are not allowed to redistribute freely.
Broadcom has not released any FOSS drivers. Broadcom doesn’t support open-source much at all.
Basically bcm doesn’t want people to use opensource.
Mediatek? Certanly not Atheros. What model?
AC3200 I believe
https://openwrt.org/inbox/toh/asus/asus_rt-ac3200_r2.34
It says in bold red not supported. Not surprising for Broadcom.
https://forum.openwrt.org/t/porting-asus-rt-ac3200-my-experience/65643
Seems what you said. Broadcom wifi chip code.
https://openwrt.org/meta/infobox/broadcom_wifi
Basically bcm doesn’t want people to use opensource.
Yeah it was some bullshit because the router is advertised as running WRT and there is an image for it, but that stupid radio chip fucked it all up.