Try this, locate the Server LUA in your DCS config folder. Make a copy as a backup. Now open the original using Notepad++. Look at line 6 which should read CameraTerrainRestriction = true. Alter the value to false, save and run DCS. You should now be able to drop through the terrain. Use Lctrl + F11 to enable free camera movement.
If you have a mod enabler you can use that.