Hi
Sometimes it can take some time to settle on a new connection.
Try turning airplane mode on and off or turn the phone off and on to help a re connection to the network.
Do you get any 3G/4g symbol in the iPhone status bar at the top .
Is Mobile Data turned on in the iPhone's settings ?
Settings -> Mobile/Cellular -> Mobile Data toggle on.
On the iPhone hold down the Sleep/Wake Button and Home button together until the Apple Logo shows. Then On seeing this Apple Logo release both buttons ,and the iPhone will reset , which has been known to clear most glitches.
Other options are :
Settings -> General -> Reset -> Reset Network Settings.
Plz try ure Sim in another phone as a process of elimination 2 rule out the phone or sim that may be at fault.
Plan you not access data everywhere , if in one place check your Vodaphone masts via
https://www.vodafone.co.uk/our-network-and-coverage/uk-coverage-map/
Current Phone >
Samsung Galaxy s²⁴ Ultra 512gb.