iPhones have always given some flexibility regarding how one can control cellular and data usage, either by disabling cellular data entirely or limiting the phone to a slower network. But why would someone do this?
Sometimes carriers don’t offer the latest generation of service in an area. For example, when the phone can’t find 4G-LTE, it will continue to search, as it assumes this is your top priority. All of that searching makes the battery circle the proverbial drain of wireless antennae. For this reason, it’s advisable to disable this faster connectivity method.
iPhones from 5 and onward have supported 4G-LTE service. At that time, the carrier parameters were changed, and the phone’s disable 3G switch (which would limit one to 2G-Edge) was replaced with a switch to disable 4G-LTE. How does this affect iPhone users? Let’s use Burlington, Vermont, as an example, where T-Mobile doesn’t offer service. T-Mobile’s network here is limited to Voice/SMS/2G-Edge. Herein lies the problem — even with 4G-LTE disabled, the phone still continually searches for 3G service.
With no option to disable 3G, the battery drains much more rapidly than in other areas. There’s nothing one can do about this, aside from jail-breaking the device to regain control over the wireless protocols. Apple has had years to remedy this and seemingly won’t…as such, the option that remains is to disable cellular service when on Wi-Fi to help preserve some battery power.