If you're like me, you're still on a years-old iPhone (12 Mini in my case!) that's getting slower and less good at holding a charge day by day. Short of keeping it in low-power mode all day, I found a way to shift away from 5G which (allegedly) uses more power for not that much of a benefit.
Yes, there's logic in iOS to try to limit drainage, but as far as I can tell, searching for 5G towers, which have a shorter range due to the physics of radio waves, is more work than with other types. For every question you might ask the internet, you'll find one group saying one thing and another refuting them, so while I can't claim that this actually helps, it sounds good to me in theory, so why not give it a try?
There are two methods for handling a switch to LTE: permanent and temporary based on your location.
Permanently disabling 5G
For the permanent fix, hit up Settings.app > Cellular Data Options > Voice & Data, and select LTE. Your phone will now ignore 5G signals while you're away from any known Wi-Fi networks. Easy enough.
The next step is to back out a level and go into Data Mode. Set it to Standard instead of Allow More Data on 5G. This latter and default setting can make your phone ignore your local Wi-Fi connection when it thinks 5G could get you your data faster. But faster isn't the issue here; we're going for lesser battery usage!
With those two settings done, you should be good to go. Keep a periodic eye on your connectivity readout on the top right of the screen. You shouldn't ever see "5G" therein.
Temporarily disabling 5G based on your location
The temporary and location-based option is a little more complicated. We'll be employing the somewhat annoyingly complex Shortcuts.app to switch to LTE when we've moved outside of a small radius around our home.
First, though, let's knock 5G down a notch so the phone isn't so apt to use it. Hit up Settings.app > Cellular Data Options > Voice & Data as before but select 5G Auto instead of LTE. As the tiny explainer text below says, "5G Auto uses 5G only when needed for performance while optimizing battery life." In On mode it will always prefer 5G even at your battery's expense. Get outta here with that mess!
Next, open up Shortcuts.app and select the Automation tab along the bottom row. Hit the plus sign to add a new one. Select Leave and then hit Choose to pick a location. Input and select your address. A map with a circle around your house will show up on the bottom portion of the screen. You can adjust this circle's radius to your liking.
With a location selected, pick the Run Immediately option so you don't have to interact with your phone to confirm the switch to LTE while you're driving or exercising. Hit Next. Confusingly, this next screen is not a list of actions but a list of premade shortcuts to start with. We don't want that, so hit the New Blank Automation button.
In the Search Actions input, type "voice" and select Set Voice & Data. This action needs to be configured by tapping any blue links therein ⸺ in this case both Radio Access Technology and Cellular Plan. Hit the latter first; without knowing your plan there will be no options for the former. Select your primary phone number from the list for Cellular Plan and Radio Access Technology should switch itself to something. It may pick LTE first or it may do one of the 5G options; pick LTE yourself if needed.
With that you've created your first shortcut. It should read naturally and as follows: "When I leave [my address] Set Voice & Data mode to LTE on Primary". Hit Done, and then do it all again. Create another shortcut but this time with Arrive as the main option instead of Leave. Similarly, this time pick 5G Auto for Radio Access Technology.
You now have two automated shortcuts. When you leave the radius of your house, the phone should switch itself to LTE, and when you come back, it'll pop back into 5G Auto ⸺ or hop onto your Wi-Fi. Pretty neat, eh? Yeah, the setup process takes a bit, but now you also know how to use Shortcuts.app, so that's a nice bonus!