it stuck’s at ios 12 and doesn’t update,
A question:
1.
Do you have a MDM profile?
If so, contact your company - IT department.
2.
Have you install / use iOS Beta version?, if so remove the beta profile (Settings -> General -> VPN & Device Management).
Then Restart your iPhone and check for updates again.
If neither applies, Go to Settings -> General -> Software Update -> Download and Install.
You need about 6 GB device storage and good Service & Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad, or iPod touch won't update - Apple Support
You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)
If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder
Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
iPhone 7
1.
Please check -> iPhone 7 Repair Program for "No Service" Issues
iPhone 7 Repair Program for "No Service" Issues - Apple Support
2.
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