The responsive layout of UBB.threads v7.5 hides sidebars for mobile users as it takes up the limited realestate to be classified as a "mobile property"; if you're not wanting to operate with a responsive layout you can disable responsive mode by disabling the viewport (CP -> Master Settings -> Primary -> Basic Tab -> Advanced Settings -> "Disable Viewport").
This will disable the viewport. The viewport element allows your forums to conform to devices with smaller screens, such as smartphones and tablets.
Checking this is recommended with any of the following scenarios:
• If your forums have non-responsive layout elements, such as in the header, footer, islands.
• If you haven't got the time to convert your design to responsive yet.
• If you have no desire to use the responsive layout of UBB.threads.
Additionally, we have several threads on this, just use the search feature and look for "viewport"