Although it is very popular and attractive to anchor Anubias to the hardscape, it is not absolutely necessary. Anubias grow just fine planted in the substrate. I have them growing in sand and in gravel.
The key is that the rhizome must not be concealed by the substrate. Once it is covered it will soon allow leaves to fall away. But I am not sure if the plant dies or becomes dormant once all the leaves fall off. I will find out here shortly as I have one in such a state. I plan on uprooting the rhizome and giving the plant some time.