Welcome to the Community Forum and thanks for your post. I'm sorry to hear that you are experiencing a problem with your 4G signal.
If you are still having a problem with this, please have a look at our online Help & Support page: Get your iPhone ready for 4G.
Let me know how this goes for you.
Virgin Media Forum Team
Need help changing SIM card? Find out more about activating your 4G ready SIM. 4G SIM Swap Help