Check iPhone type -> Go to Settings -> General -> About.
iOS 12.5.5 is last security updates for iPhone 5S / 6 / 6 Plus.
Try first 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
Try also to optimize the Wi-Fi network, see the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
See this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support
Low Power Mode is On.
Charge the iPhone & wait until the battery is fully charged (100%).
The Low Power Mode will turned off automatically.
Then you can perform the update, if your iPhone in not running iOS 12.5.5 yet..