Try hard resetting a couple more times. If that doesn’t help, try resetting your network settings. Settings > General > Reset
You’ll need to re-enter any saved WiFi passwords.
You could also try updating through iTunes.
If all else fails, you can try a reset all settings. Settings > General > Reset
This won’t erase any data, but it will put your iPad back to factory default settings.
Last resort would be to wipe the iPad, restore from an iCloud backup and it should automatically ask you to upgrade to the latest iOS (11.2.5) during setup. Make sure to do a manual backup before restoring if one hasn’t been done in awhile.
Good luck!