changed the phone from 8 to 13 pro, the connection is terrible, mobile phone calls and video calls over wi-fi

You need to Contact your Carrier, please. WiFi Call is a Carrier Specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.

It seems you do not have a good decent internet connection.

For your Internet connection.

• Over Cellular: contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you
• Over WiFI: Contact your ISP (Internet Service Provider), the agency who provided the Internet Service on your Modem/Router to you

However --> Connect iPhone to the Internet - Apple Support

Check your iPhone settings

Go to Settings and turn on Airplane Mode, wait five seconds, then turn it off. Check Do Not Disturb. Go to Settings > Focus > Do Not Disturb and make sure it's turned off. Check for any blocked phone numbers.

If you can't make or receive calls on your iPhone - Apple Support

Can't send or receive message then try these steps first

1. Restart your device: ... 
2. Check your network connection. ... 
3. Check with your carrier to see if the type of message you're trying to send, like MMS or SMS, is supported.
4. If you're trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging.

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If you can't send or receive messages on your iPhone, iPad, or ...