Is the smartphone on your wifi? If it is, just follow the steps to add your whole subnet.
If you’re trying to access it while using the phone’s network, that’s going to be harder. You’re going to have to configure your router to allow port forwarding to your pi, then figure out what your phones IP is. Then you could be safe and only allow that IP. The problem with that is when your IP changes, you’ll have to edit your whitelist again. You could try to add the class C or D subnet if you don’t want to worry too much about this, but you’ll be opening up your network to the internet in the process so good luck!