iPhone 6+ 64 gig, running on MTS (MB-Canada). I have had numerous issues with total loss of internet (but not cellular). Doesn't matter if I'm in the city or in the surrounding countryside (I live about 30 miles out of the city, but there's still LTE there).
I often use Siri when on the road to do voice-to-text etc over my car's bluetooth system, and I'll often hear, "Siri not available. Connect to the internet". There's been times where my girlfriend, using an iPhone 5C on the same carrier (MTS), is sitting right beside me and she has full LTE, no interruptions. Her phone can stream HD quality video while she's riding shotgun down the highway without ever a hiccup, but my phone keeps losing internet.
At my office in the city, my phone often falls down to Edge or GPRS connectivity, which is to say, utterly useless (can't even check text-only emails). I've been to the Apple store in town, to the Genius bar. They did a full factory reset twice on the phone. The carrier has replaced the SIM, and verified all network settings. I've done network resets, and rebooted the phone frequently. Toggling airplane mode on/off sometimes restores the signal, but usually I just wait a minute or two and then I get a zero or one bar LTE again.
Apple store said the next step was to swap out the phone, but given what I've read here, I doubt that will help. I love the iPhone, love Siri, so I'm going to wait it out for a bit longer. I came from Android to the 6+ because my experiences with Android phones were utterly frustrating.
Please, Apple, if this is just a software issue for the 6+, get it fixed! If it's hardware, figure it out so we can get our 6+'s replaced with "fixed" models in-store while they're still on warranty!