Wifi is not working after installing MIUI
I had exactly the same problem with my Xperia M (C1905) and Miui 4.9.26.
It turned out that the cause was that I didn't wipe the old stock android.

So the solution was:


enter TWRP
advanced wipe and wipe everything except sd card (I also didn't wipe the last item USB something).
Install MIUI again