First of all, you need to save the "connection" that you want to assign a static IP Address.
Then Go to "Networks and Connections".
Then to "Wi-Fi Network".
Then to "Saved Wi-Fi Networks".
Choose the network you want to assign a static IP Address (The Network that you have saved before).
Uncheck the box next to "Automatically obtain IP address and DNS" to show the static IP Address form.
Enter your networking information (IP Address, SubNet Mask,Router, DNS).
Hit the "Save and Connect" button.
After this, you should connect to the network you want.
I hope this guide would help.