Why does my IP Address change?
If you are connecting to your WiFi using D.H.C.P. your IP Address for your device can change each time you connect / re-connect to your network.
When you switch from WiFi to mobile data your IP Address will change.
If you disconnect from one WiFi network and connect to a different WiFi network your IP Address will change.
Why can’t I input an IP Address?
Your device is assigned an IP Address by the network you are connected to.
You can change this but you will have to do so manually on your device. This page provides an example of how to do so – https://www.lifewire.com/change-ip-address-on-android-4588283