1.
Try to free up iPhone device storage see this Apple articles below:
If you need more space for an update on your iPhone, iPad, or iPod touch
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
How to check the storage on your iPhone, iPad and iPod touch
How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)
Clear Other storage on your iPhone or iPad
Clear Other storage on your iPhone or iPad - Apple Support
2.
If you wish, you can upgrade the 5 GB free iCloud.
See -> What's the difference between device storage and iCloud storage?
What's the difference between device storage and iCloud storage? - Apple Support
See also the following Apple articles:
Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support
iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support
How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)
Manage your iCloud storage -> Manage your iCloud storage - Apple Support
3.
Try to optimize the range of the mobile network, see Apple articles below:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)
If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart
If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support
4.
Charge the battery > 70%.