After more research I found:
and in this page find line “Special Case #2 - For the ASUS PCE-AC88 AC3100”
and download this driver brcmfmac4366c-pcie.7z
unzip and copy into /lib/firmware/brcm/ folder on the LEDE router, reboot and everything worked. This driver has an August 18, 2017 timestamp so it should be a new one.
My router has LEDE 17.01.1.
Looks like I was lucky.
PS. Also I came across this info on another forum with an older driver, it might help someone