How to get service on iPhone 5s?

Hi, i recently bought a new iPhone 5s. My old phone that i still have with its sim, is an iPhone 4. I have to change sim from a micro to nano sim, but i have no service on my new phone, so i can't call anyone. I'm with O2. Also, wheres the 4G! I have no 4G anywhere, not even in Central London. I live outside london though. The only way i can go online is through Wi-Fi! Please help, I'm seeing O2 tomorrow but i was just wondering if there was a simple solution. Some one said i have to call O2 to activate my sim card?

1. Walk several yards away and see if the "No Service" message disappears. If it appears only in a specific geographical area, you have discovered a cellular dead spot, which has nothing to do with your device.
2. Hold down the iPhone's "Home" and "Lock" buttons until the Apple logo appears on its screen. Wait for the device to reboot and see if the "No Service" error message is still there.
3. Insert an unfolded paper clip into the hole next to the SIM card tray, which sits on top of the iPhone 3G and iPhone 3Gs, and on the right side of the iPhone 4. Push in gently until the tray pops out and remove the SIM card. Blow gently on the SIM card to remove any debris, then re-insert the SIM card face down and replace they tray. Wait for the "No Service" error message to disappear.
4. Tap the iPhone's "Settings" button, then tap "General" and "Reset." Tap the red "Reset Network Settings" button to reset the iPhone's network. Reseting the iPhone's network settings clears out its memory, which removes any device-related errors that could cause a SIM read error.
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