I'm having the exact same trouble with my two, same AP6181. I've tested both Xenial (3.4) and Stretch (4.19) but neither have Wifi working. The older kernel image is loading dhd drivers but its a repeat of what Andeser is getting. Stretch says dhd module not found. using "sudo modprobe brcmfmac" on either kernel adds "registered new interface driver brcmfmac" to dmesg but nothing else.