Changing the country code should be sufficient: detailed steps at wifi_countrycode
Ticket 9678: ar71xx: DE locate, but channel 12 and 13 are disabled wontfix This problem cannot be fixed in OpenWrt due to various obligations and agreements with vendors.
Wikipedia says "Most countries in world" allow channels 12 and 13
Workaound Solution: http://luci.subsignal.org/~jow/reghack/ + set your country in
OpenWrt Forum discussion to enable channels 12 and 13
Note: due to r31954 tweaking crda isn't longer an option.