Is 4G set to On in your iPhone. Settings > Mobile data > Enable 4G > Select Voice & Data.
Are you in a 4G area.
Do you get 4G connection anywhere ?
Can you try your Sim Card in another phone as a process of elimination to rule out the phone or sim card that may be at fault. Try another Sim in your phone. If necessary any Vodafone Shop can replace the Sim Card via a Sim Swap.