Hey there gracel96,
We'd like to help answer your question as best we can. Regarding the signal changing from 3G to 4G, this is automatic, and depends on a few things: cellular strength, cellular coverage for 3G and/or 4G, if your iPhone is capable of receiving both kinds of signals, etc. We'd like to learn more about what's happening, along with ruling out some basics:
- What exactly is the issue you're having with your cellular signal? The more details you provide, the better we'll understand what's happening.
- If there's a technical issue occurring, have you already contacted your carrier about it? A lot of the factors we listed above can only be confirmed by your carrier.
- For your iPhone model, the latest version is iOS 15.7.9. You can verify this within Settings > General > About. In case you needed it, this link shows how to first back up, then update: Update your iPhone or iPad We also recommend checking for any cellular settings updates with these steps: Manually update your carrier settings on your iPhone or iPad
Cheers!