That´s precisely why the composition of an exoplanet is unknowable.
There are 2 observables - the total mass and volume. There are 4 major variables of internal composition: the amount of iron, the amount of rock, the amount of water and the amount of hydrogen/helium. Remember that 2x Earth radius and 8x Earth mass means twice Earth escape speed, and that is in the region of what Uranus has.
If a body consisted of only 2 variable substances, like iron and rock, or rock and water, or water and hydrogen, then you could solve for an unique composition that fits the observed mass and diametre. If you have 3 unknowns then you cannot solve, because it is underdetermined equation system, and you have degenerate solutions. A body with tiny iron core and mainly rock, like Moon, can have the same density as a body with a big iron core but also a large amount of water on top. And as stated, you have 4 unknowns, not even just 3.
Lets remember that Uranus has a density of only 1.32 a radius of 4x Earth and has a small rocky core, water/ice mantle and hydrogen rick atmosphere. I would think that a planet with a density of 5.52 with 2x Earth radius would have most of its mass made into a iron core, rocky mantle and crust while being much more water rich than Earth. I mean take Mars for example it is made of less dense materials on average.