UPDATE: I deleted my earlier post because after being up for several hours, the FireTV Cube started working with Alexa smart home camera skills including Monocle.
I received the Amazon FireTV Cube today (2018-06-21) and at first it was not working with Alexa smart home camera skills. However, after leaving it up for a number of hours and re-testing its now working with the Monocle camera skill to show my IP camera feeds.
NOTE: However just like the FireTV 4K (3rd Gen), the FireTV Cube does require using the new Monocle Gateway service which is currently in BETA testing.