If Apple Service said there is nothing wrong with the phone then you can’t claim that it needs repair, which would be the first step in a warranty repair.
As a first step: Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN. However, VPN actually reduces your privacy, it does not enhance it.
If you don’t have a VPN app or profile installed, or removing the profile doesn’t fix it:
- Open Settings
- Tap General
- Tap Transfer or Reset Phone
- Tap Reset
- Tap Reset Network Settings
- This will erase all Wi-Fi passwords, but will not result in any other data loss.