Hello
I've registered and posting this to help others. My Mazda 6 2015 (facelift) SE-L Nav would not connect to a WiFi network. I tried every suggestion from everywhere I could find (changing passwords to certain lengths, different networks, different channels), none of which worked. It seems that the CMU is faulty and this appears to be a known issue to Mazda Europe - the dealer spoke to the Mazda Connect people in Germany. The only option is to get the CMU replaced. As longs you have updated your SD card to the latest version (don't ask me why this should be a requirement) and say you are prepared to buy Connected Services and show that you've tried multiple WiFi networks (that already work with other devices), there doesn't seem to be any resistance from Mazda to replace the CMU, which suggests they are well aware of the problem.
I hope this saves some frustration for other people.
Peter