My iPhone is currently unable to access so cellular data or make and receive calls or messages

iPhone 7

1.

Please check -> iPhone 7 Repair Program for "No Service" Issues

iPhone 7 Repair Program for "No Service" Issues - Apple Support

2. 

See -> If you can't make or receive calls on your iPhone

If you can't make or receive calls on your iPhone - Apple Support

3. 

iPhone 7 is > 6 years old, which also affects the battery life. The hardware remains old, so does the battery.

If it is still possible, have the battery replaced.

The battery can be replaced by Apple or an Apple Authorized Service Provider.

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE