WiFi Channels 12 and 13 on Asus ZenPad S 8 and Sony Xperia S not visible

I bought my daughter a Asus ZenPad S 8.0 (Z580C). Wonderfull android tablet with Android 5.0, but one day her WiFi stopped working.
Actually the wifi worked fine, just my home networks were not visible.
Turned out the the ZenPad just did not work on channels 12 and 13.
I turned off channels 12 and 13 use on my WiFi router and the problem was solved.

I found no options on either the ZenPad or XperiaS to enable those channels as yet.