Well had a bit of luck after spending all day on this.
Arduino UNO (sender): Like mentioned by chrisl, use Serial.write and not Serial.print
mySerial.write(sonar.ping_cm());
UNO Wifi (receiver):
Serial.println(mySerial.readString());
So, instead of mySerial.read I used mySerial.readString.
This is most likely a obvious mistake to almost everybody here; took me two days. Sigh.
