Google has reportedly fixed the data connection problems for some Nexus 6 users, as they have recently posted on the Google Product Forums that after having taken in plenty of user feedback they have pushed out a fix that should resolve the issue. According to Google, GPS was causing the data connection problem, more specifically it was causing cellular connectivity to fail. Although the post on the Google Product Forums seems to have been created on August 3rd, the issues were more widely reported on August 5th just a couple of days later when more and more users started to experience the same problems.
While it's entirely likely that not all people who have had this issue with cellular data connectivity will notice a fix, Google's efforts should be extended to any users who haven't been able to connect to data through their network. If however there are still problems that are surfacing, Google recommends that users reboot their device and see if the fix takes hold afterwards. Since the fix was something that Google implemented on their end of things, users won't have to worry about updating their device with an over-the-air software patch, which is why the device restart should force the fix to come through if it doesn't beforehand.
A few users have reported that they have experienced this problem on Nexus 6 devices on networks other than T-Mobile, but it appears that the cellular data connectivity failures were mostly limited to T-Mobile's network. Aside from Google's fix that was pushed out this morning, some users have apparently been able to remedy the issue on their own by one of two methods, including updating the Google Connectivity Services through the Play Store or by altering the APN settings of their device, something which T-mobile may have been able to assist with. It's also worth noting that some users are now reporting that they can once again connect to data on their network, but are being met with slower than expected data speeds even though the indication in the status bar is that they're connected to 4G. Having said that, it is possible that other users will experience this slower data speed as well, and it's also possible that the reboot could fix this problem.