When my sister, mother and I are in the same place (for instance, riding in a car), their iPhone 6 phones show LTE coverage whereas my iPhone 6 is receiving only 3G. (And my work iPhone 6 phone shows LTE, so really we're talking 3 LTEs and 1 3G all in the same car). Does anybody know why?
One could be someone turned off the radio. The radio may be faulty. Have the person with 3G go into settings and reset network connections. It'll restart and you will not lose any personal data.
The iPhone 6 and 6 Plus will hang on to 3G longer than previous models. I suspect Apple designs this to reduce frequent switching between 3G and LTE in the setting of a low LTE signal to prevent wasted battery. My iPhone 6 will do this if I was in an area with low LTE signal and it switches to 3G. It will then stay on 3G a while even when I get to an area of good LTE. In some cases in takes 5 minutes to switch back to LTE. I notice this when I am in an interior of a building for a while where there is minimal to no LTE signal but acceptable 3G signal and then go out to the parking lot where there is good LTE signal. I will be driving 5 minutes before it decides to switch back. If I turn airplane mode on for a minute and then turn airplane mode off, it will force it to recheck for LTE and connect faster.