system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why 3G Is Worthless On My LTE Phone, But 3G Was Fine On My 3G Phone

I have an LTE/4G phone which while receiving an LTE signal is fast and fine. However, sometimes it drops to 3G coverage and data is almost unusable.   Up until about 2 months ago I was using a 3G phone with no LTE capabilities. While it wasn't as fast as LTE, I also had no complaints, it worked.   
So why does my LTE phone become near useless in the same 3G only coverage areas? 

Edit: Verizon, iPhone 4s to 5s, in case it matters.
gpt
In the default mode, the phone will prefer a 4G network over a 3G network, and any time it drops to 3G, it will begin searching for a 4G network to connect to instead.  Depending on how well that particular search logic is implemented, it may end up expending a lot of resources in a fruitless search for a signal that just isn't there, or is just barely there, resulting in very high signal failure rates (where the signal is just good enough to detect its existence, but not good enough to function, which then requires multiple attempts to send the same data).  If your phone has the option to select the radio mode, try turning it to 3G instead of 4G/3G.  Disabling 4G entirely will give you a much better point of comparison, and probably a better result as well.