IBus
Set up OpenBangla Keyboard on LXQt using IBus.
Select OpenBangla Keyboard
In the small window, click the dotted vertical line option and search for bangla. An item named Bangla should appear, so click on it. Scroll to the very bottom, select OpenBangla Keyboard, and click Add.
Switch to OpenBangla Keyboard
In the bottom right there is an IBus panel icon. Click it and change to Bangla - OpenBangla Keyboard.