I have had this problem since the iphone 6+ first came out. I believe that was IOS 8. In fact I had this problem on an iPhone 5 which is what I used prior to getting the 6.
Its getting out of hand. The iphone 6 wifi/lte driver stack is unrefined and unpolished. Right now I can't connect to anything. What I will notice is that sometimes it happens when I am in my driveway leaving for work. What happens is I fire up a music app like Pandora or Apple Music and I have connection issues. I end up playing music from my phone library and then I fire up Waze. I get driving directions but the infamos "connecting to network" red text at the top along with no ETA. Today I even had a pop up to log into iTunes store for no reason at all (I was driving so quickly hit cancel).
It's almost like it doesn't work properly when switching from Wifi to LTE, which it should be able to do. It gets stuck in some kind of limbo state where it just will not connect to anything. I've done things like cycle airplane mode, cycle LTE data, wifi off and on, etc... It won't come back until you reboot the phone. My Windows 10 PC that is hardware more than 4 years old boots faster than IOS on an iPhone. No one wants to waste time rebooting the phone.
I have the latest IOS 8.4 and there are no further updates so its not like I can even do anything about this problem.