For me, the problem seems to come from the system trying to handle multiple signal sources, because when I have both wi-fi and cellular data enabled, I get exclamation marks on both the wi-fi and cell-bar icons. So when I am home, where my router is always on, I turn off my cellular data (in data settings) and rely strictly on the wi-fi signal. I do the same if I am using a strong wi-fi signal elsewhere. I only enable cell data when I actually need it and have no decent wi-fi available. The exclamation marks on my Moto-g/ Android 5.02 don't seem to signify a serious problem (because I still get internet, even with the exclamation marks) - but it seems to mean that there is some struggling going on within the operating system as far as deciding on a proper signal source. My guess is that some of these apps on my phone aren't properly coded for handling background data via multiple signal sources. So when you see exclamation marks, just disable your cellular (i.e., mobile) data transfer and re-connect your Wi-Fi (or vice-versa). The exclamation marks should disappear if you are limiting your signal source to one or the other. Just keep a link to your system settings on the home screen, and the switch just takes a couple of taps.