Hey vichadan,
Welcome to Apple Support Communities. The information below may help with your issue.
Have you tested with a different Wi-Fi network? If not, we recommend doing so. It will help with isolating the issue by ensuring it isn't network related.
Make sure the date, time and time zone is correct on the iPhone. You can check the time zone in Settings > General > Date & Time. If the time zone needs to be updated, follow the steps in this articled to do so: Change the date and time on iPhone.
If there is a VPN enabled and/or security software installed they may be interfering. Remove or disable and then attempt the back up again. For more details about how VPN and security software may cause network connectivity issues, click here: Check VPN (Virtual Private Network) and third-party software to help resolve network connectivity issues .
Best wishes.