Archer A6 V3: 5GHz stopped working, failing with EIO

I flashed Archer A6 V3 with OpenWRT 21.02.1 originally. 5GHz wasn't working. I got it to work by setting the channel to 36. Then I started tweaking other settings in Luci (like WPA3, power etc), and the 5GHz radio stopped working. I was unable to restore the settings to make 5GHz work again, but I haven't tried a reset to defaults.
Then I upgraded the router to the December 3 snapshot, r18270-fc4398fe71. 5 GHz is still not working. The network device wifi1 is down. If I try to bring it up, I get EIO.

# ip link set up wlan1
ip: SIOCSIFFLAGS: I/O error

I discovered that the kmod-mt7615-firmware package wasn't installed, so I installed it with opkg, but it made no difference, even after reboot. There are no error messages in the kernel log when the driver if reloaded and an attempt is made to bring the interface up.

root@Archer6:~# rmmod mt7615e
root@Archer6:~# modprobe mt7615e
root@Archer6:~# ip link set up wlan1
ip: SIOCSIFFLAGS: I/O error
root@Archer6:~# dmesg | tail -n 5
[   21.205584] IPv6: ADDRCONF(NETDEV_CHANGE): br-lan: link becomes ready
[   23.075785] mt7530 mdio-bus:1f wan: Link is Up - 1Gbps/Full - flow control off
[   23.083029] IPv6: ADDRCONF(NETDEV_CHANGE): wan: link becomes ready
[ 1105.096121] mt7621-pci 1e140000.pcie: bus=2 slot=1 irq=23
[ 1105.112859] ieee80211 phy2: Selected rate control algorithm 'minstrel_ht'

The only message containing mt7615 in the kernel log is:

root@Archer6:~# dmesg | grep -i mt7615
[   11.712050] mt7615e 0000:02:00.0: enabling device (0000 -> 0002)

Country is set to US on both WiFi interfaces.

Full kernel log and /etc/config/wireless are attached.

dmesg.txt
wireless.txt

Start with the attached dmesg.txt and wireless.txt, then inspect the mt7615e initialization path for the wlan1 interface. Reproduce the reported ip link set up wlan1 failure and compare the kernel output with the wireless configuration. Done means the 5 GHz interface initializes and can be brought up without EIO.