Here are several things you can try to fix slow cellular data internet speed on your iPhone:-
1. Look for strong signal bars:- The signal bars on your iPhone indicate cellular reception strength. More bars generally mean a faster connection. If the bars are low, try moving to a different location with better reception.
2. Switch between Wi-Fi and Cellular:- Briefly turn off cellular data and connect to Wi-Fi (if available) to see if the slowness is network-related. If internet speeds improve on Wi-Fi, the issue might be with your cellular network provider or signal strength in your area.
3. Simple Restart:- Sometimes a quick restart can resolve temporary glitches. Turn off your iPhone completely and then turn it back on.
4. Low Data Mode:- This mode reduces cellular data usage by limiting background app refresh, automatic downloads, and high-quality streaming. Go to Settings > Cellular > Cellular Data Options and turn on Low Data Mode.
5. iOS Updates:- Ensure you have the latest iOS update for your iPhone, as it might include bug fixes or improvements to cellular performance. Go to Settings > General > Software Update.
Follow for further instruction below.
https://thegeekpage.com/slow-cellular-data-internet-speed-on-iphone/