How do I fix slow and unstable internet connection on my iPhone?

What you can do:

• Restart your iPhone
• 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. 
• 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.

Beyond that, contact your carrier, as cellular data is their responsibility

Well, there are hundreds of possibilities based on the information you have provided so far; let’s try to narrow it down; Most important: Is this a Wi-Fi connection, or a cellular data connection?

From there we can begin to troubleshoot.