I recently received a second device and was having issues getting devices to associate with the second 5GHz radio. After some investigation I noticed that the MAC was the same on both devices.
I tried various configurations of setting the mac address in /etc/config/wireless and in /etc/config/network but nothing worked. Eventually I had to rewrite the factory partition.
I added 1 to the last octet in the mac address and rebooted, which finally worked. Here's what I did:
I'm pretty sure that getting this wrong will brick the router, so be careful!
# Make the mtd device writeable:
# opkg install kmod-mtd-rw
# insmod mtd-rw i_want_a_brick=1
# mtd unlock /dev/mtd2
# dd if=/dev/mtd2ro bs=131072 skip=0 count=8 of=/tmp/factory-fixed
# The MAC addresses are stored in bytes 4 through 9 for radio0 and 10 through 16 for
# radio1. You can view then with:
# dd if=factory-fixed iseek=4 bs=1 count=12 | xxd
# Update the last byte in of radio1's MAC:
# printf '\x4f' | dd of=factory-fixed bs=1 seek=15 count=1 conv=notrunc
# Write the partition back:
# mtd write /tmp/factory-fixed /dev/mtd2