AIXI can't find itself in the universe - it can only view the universe as computable, and it itself is uncomputable.

It can't exist in the universe either.

Computable versions of AIXI (such as AIXItl) also fail to find themselves in most situations, as they generally can't simulated themselves.

What do you mean by "find themselves"? They are likely to naturally develop a model of their location, their sensory capabilities, their motor capabilities, and their cognitive capabilities. They won't contain a complete model of themselves - but then, no agent can ever do that.