I found that reliability of smartphones depends more on the reliability of the service provider, not the hardware itself which is usually more than adequate for most customers.
At home, I always use my wi-fi connection with my iPhone. Spectrum is the same internet provider I've had for the 23 years I've lived here. The problem with smart phones, and possibly with the older iPhone like mine, is that they have a smaller antenna than a regular cell phone. When this one dies, that's it for me. I can't see paying large sums of money for something I basically only make calls on.