To install St 5.11.2 on Ubuntu 18.04, get the Qt installer file from the Qt download page:
wget https://download.qt.io/new_archive/qt/5.11/5.11.2/qt-opensource-linux-x64-5.11.2.run
Make the file executable:
chmod +x qt-opensource-linux-x64-5.11.2.run
Execute the file:
./qt-opensource-linux-x64-5.11.2.run
You should see the Qt installer:
Follow the steps in the installer. Once the installation is completed, Qt Creator should appear in applications list.