If you're connected to wifi, you'll have the external IP of that network. So you could simply disable your wifi and get the IP from your mobile provider or connect to a different wifi.
Not necessarily. The IP assignment and frequency of IP changes depends on how the ISP assigns IP addresses to devices on their network. Your best option is to check the IP on your mobile device, turn it off and back on again and see if the IP has changed. My suggestion would be to perform this task during a busy time of day. If you turn off your device, someone else might be turning theirs on and be assigned your IP. Once you turn your device on, you may be assigned a new IP since your old one was taken.