Sri Ramakrishna never called himself an Advaitin. These systems are what he considered to be true.
'Again I cannot utter a word unless I come down at least two steps
from the plane of Samadhi. Sankara's Non-dualistic explanation of
Vedanta is true, and so is the Qualified Non-dualistic interpretation
The Gospel of Sri Ramakrishna, Chapter 38, With the Devotees in Calcutta, March 11, 1885
Why did he consider Saguna Brahman real?
'You see, in one form He is the Absolute and in another He is the
Relative. What does Vedanta teach? Brahman alone is real and the world
illusory. Isn't that so? But as long as God keeps the 'ego of a
devotee' in a man, the Relative is also real. When He completely
effaces the ego, then what is remains. That cannot be described by the
tongue. But as long as God keeps the ego, one must accept all. By
removing the outer sheaths of the plantain-tree, you reach the inner
pith. As long as the tree contains sheaths, it also contains pith. So
too, as long as it contains pith, it also contains sheath. The pith
goes with the sheaths and the sheaths go with the pith. In the same
way, when you speak of the Nitya, it is understood that the Lila also
exists; and when you speak of the Lila, it is understood that the
Nitya also exists.'
The Gospel of Sri Ramakrishna, Chapter 42, Car Festival At Balaram's House, July 14,1885