iphone5s keeps locking up every few seconds
how to keep iphone 5s open for several minutes
iPhone 5s, iOS 12
Check: Settings>Display&Brightness>Autolock... and choose the time there.
If it doesn't work, restart your iphone and try again.
If the issue still persists, your iphone could have a hardware problem. Have it inspected at an Apple store or Authorized Service Provider: Apple – Location Selector
Regards
Giulio