Is there a way to make the Enter key work with a Logitech K811 keyboard?

If you don't need to use fn otherwise, you could simply remap it to behave like Enter. This is easily doable using the free tool Karabiner-Elements — download and install it, then add a "Simple Modification" from fn to keypad_enter. If you want to only remap your external keyboard, then set the checkboxes in the Devices tab appropriately.

If you want to keep fn's functionality, you need a more advanced remapping/macro utility such as BetterTouchTool or Keyboard Maestro. I'm not aware of any free options in this realm (though they may exist) besides the full version of Karabiner, which doesn't currently work on Sierra.

fn is not a typical modifier key, so it can't be captured in the same ways that the others (e.g. ⌥ and ⌘) can. Because of this, there is no way I'm aware of to assign fnreturn to anything. However, using one of the tools I mentioned, you can easily assign another hotkey to do the same thing, for example, ⌥return.