It’s a long-shot but won’t hurt to try it – see if you can add the location of your help to IE’s list of Trusted Sites (Tools > Internet Options > Security tab > Trusted sites > Sites button)
I managed to solve the issue. It is a setting issue in IE. In IE, go to Tools > Compatibility View Settings > select the Display intranet sites in Compatibility View. By enabling the option, I can now see the table of contents in the left pane of the online help file.
But this is only a solution for a single browser installation. All other readers will have the same issue. If you want to support IE11, the only real fix is to upgrade RH.