Try to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
If you still don't have a 'service' after that, see -> iPhone 7 Repair Program for "No Service" Issues
iPhone 7 Repair Program for "No Service" Issues - Apple Support
i have also tried to use my hotspot
For update, if they are On, turn Off Hotspot & VPN.
it gives error code 1667.
See this Apple article: iOS update and restore errors
iOS update and restore errors – Apple Support (UK)
⤵️
Check your hardware
If you see error 53, use these steps.
If you see one of the errors below, you may have a hardware issue:
- 1, 3, 11, 12, 13, 16
- 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 56
- 1002, 1004, 1011, 1012, 1014, 1667, 1669
Here's what to check. Try again after each step:
- If iTunes is already open, quit and reopen it.
- Check your third-party security software.
- Make sure your USB cable, computer and network are reliable. Then try twice more to restore your device.
- Check for outdated or modified software.
If you still see the error, contact Apple Support.
Then try again to update to ioS 15.6
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
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
You need about 5 GB device storage and good Wi-Fi.
