Using a 3G sim in a 4g phone

[Pauleemillwall]

My phones playing up iPhone 5c I got it relpaced yesterday for a new handset in Apple store but I'm using a 3G sim in a 4g handset why is my new phone messing up now again

 

[Titillating]

Can you clarify your question? Not sure what it is you're asking right now.