Adding to the woes, some Nexus 6 users are now reporting that they are experiencing failed mobile data connections. Based on user reports, the issue has been seen in multiple users on different carriers. Preliminary reports hint that the issue has nothing to do with Android 5.0 or Android 5.1 Lollipop updates. It starts off with "!" logo in the signal bar which means the connection (3G or 4G) has been dropped and will not connect on its own. On restarting the device, the problem gets fixed for some time; though it goes back to the same situation.
One of the users on XDA Developer forum said, "This last week at work I started noticing that my data and voice would drop off and I would get the icon showing half signal but would have an ! In the icon, when I look at the Sim status it shows I'm connected but I'm not. After rebooting the phone I will have 4G services for about 20 minutes then it drops off again. I have Tmo [T-Mobile] and contacted support and they told me to reset the APN but that didn't help. The only thing that looks new on my phone is a recent app called Google Connectivity Service."
Another user Reddit wrote, "For the past week or so, my phone's connection switches over to have "!" connection, even if full bars. Doing a soft reset fixes the issue, but the issue happens again after a few hours or so." The issue was first reported by Android Police.
One of the biggest advantages with the Nexus range is whenever an issue hits the device, Google acknowledges it and releases fix soon. However, in this case we are yet to hear any word from Google.
It's also worth mentioning that very limited number of Nexus 6 users have been affected with majority of them running T-Mobile and Sprint networks.