Sim cards don't control signal levels, these are determined by
1. The Antenna in the Phone
2. The algorithm in the software on the phone.
Have you tried the sim card in a different phone?
Have you checked https://status.o2.co.uk to check if there are issues in your local area?
You could try dropping down to a 4G connection (or LTE) if that is in the phone menu @Melissa_jayne96 ?
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here