In order to get a camera to be fixed in position, but point at a moving target, make a new blueprint of type Actor. Add a component Camera and then in the Event Graph edit the blueprint like theimage below.
I called mine LookTarget, – this will be the moving thing we want the camera to point at.
Drag the compiled blueprint into your level and in the details panel look under default and you should see Look Target – click the eyedropper and select the object you want to track in the 3d viewport.
In order to set this camera to take over from the default, you need to have a look at this tip here but make the reference to <yourcamera>.
This is all thanks to Giuseppe Portelli and his great post here.