[Nexus 6 Q&A, Help & Troubleshooting] Skreelink:
After searching extensively on the whole Nexus 6 signal debacle. I've gotten some observations and studies to share and a couple questions.

I recently obtained a Nexus 6, which I already knew had the ! no data connection problem. (I tried to help the previous owner with the issue before, calls/texts would work, but data wouldn't) They were naturally on T-Mobile, so I assume it was related to the project Fi launch. Googling the issue recently states mostly T-Mobile and Sprint users having the issue, so I decided eh can't pass up a good deal! A like-new Nexus 6 for only $70, who can resist? I figured even if I get the no data issue, I can just turn on airplane mode, use as a mini-tab.

Here's where it gets interesting: I use Cricket (which is AT&T of course), popping the sim in results in good signal, LTE, etc as expected. Randomly (sometimes within 2 minutes) instead of the "!" on service problem, it drops to No Service completely. A reboot and it comes back fine, this wouldn't be too much of an issue except it happens EVERY 2-10 minutes. The only way to get it back is a reboot, airplane mode toggle doesn't help.

Which leads into the second part of the conundrum: Airplane mode doesn't help.

I figured I might just keep it in airplane mode and use as a tablet... The phone said no, Was fully charged before going to bed, by morning 17%. In airplane mode no less, which is of course is abnormal. Looking into it, it seems the phone never went into Deep Sleep, which I immediately delved into the cause. At this time I was running CM13 latest nightly, naturally I went for a fresh stock flash... Same issue. I even stock flashed to 5.0.0 and the same issue... Looks like hardware, but the randomness is the confusing part and you'd think if it was hardware a simple reboot wouldn't fix.

The phone never slept because when it would "No service" on me, I'd get wakelocks for "msm_hsic_host" which would keep the phone awake EVEN IN AIRPLANE MODE, which also in the battery settings showed the red bar for "bad" signal. This happens with AND without a sim, airplane mode or not. I'm currently on Pure Nexus and Franco's kernel to disable hsic wakelocks, and I'm seeing another one for "qcril_pre_client_init" when the issue happens. Which would be the Qualcomm radio interface layer, by the name, I'm assuming android is trying to turn on the radio and keeping the phone awake. This shouldn't happen in airplane mode, I've also tried "Toggle Cell Radio 5.0" to directly turn off the cell radio; Issue remains.

The randomness of when it happens, how it's "resolved" by a simple reboot, and the fact that airplane mode has zero effect on when it happens basically cripples it as a tablet as well, it never sleeps so the battery life is pretty crap (about 18%/hour screen off). After a reboot the thing is great as long as the thing has signal, runs cold, great life, charges fast, sleeps deep. Befuddled that it still does it without a sim card and airplane mode...

My question to the users here: If anyone else has any insight on this, and if anyone knows how to completely disable the cellular interface and literally turn it into a tablet. Is there any rom or such that doesn't try to wake/use the cellular radio and keep the thing awake.

EDIT: Yes, I've run the factory images from google, 5.0.0, 6.0, and 6.0.1 during testing.

The sim card is new, I've already changed it.

Yes the device is actually an XT1103, not a knockoff.

Sorry if any of this information wasn't clear enough.

Further Observations:

During the night I had zero reboots (It's setup to reboot on signal loss, so from here reboot == signal loss), then after taking it off of charge and turning the screen off, then back on after about 5 seconds the signal was lost and it rebooted.

This got me thinking, so on the way to work, I streamed music and had zero problems, not on the charger. After reaching work, I moved some offline music onto my phone and it acted fine as well. The phone never reboots if the screen is kept on as well.

This tells me it's something about how android switches to low-power on the cellular radio. Perhaps the radio cannot wake up properly from the low-voltage sleep state.

tl;dr

When phone is 'active' and not sleeping, problem doesn't arise.

Acts normal with screen off: Streaming music, Regular music, Plugged in.

Acts normal with screen on always.

If doing nothing and phone sleeps: reboots, usually on wake up (i.e. turning screen on to check time)

Likely issue with sleeping/low-power to the cellular radio.

Current bad "solution" play music with phone muted.

However, this doesn't seem to be too bad an impact on battery life, music player has used a total of 1% in the last two hours, screen is top again, where it was android os.

EDIT: Went all day without dropping signal by keeping music playing muted. At 52% after about 12 hours, 3 hours screen on, an hour VOIP call, wifi on the entire time. Not the best solution, but it's livable.

Further testing: I've tried a couple "keep awake" apps that set wakelocks and they do not seem to help, but this might be that they are not designed for marshmallow.

I've also "unoptimized" anything related to phone/cellular in the battery settings to prevent doze from sleeping them, no effect, double the apps directly hit the radio.