According the AD9958 datasheet the specifications of the device are provided for a DVDD_I/O voltage of 3.3V+-5%. However it is not explicitly specified the minimum DVDD_I/O voltage, but my guess is that it is 3.3V -5%.
In my design I incorrectly set the DVDD_I/O voltage to 2.8V and the SPI interface is not recognizing the commands so I guess that this is the root cause of the issue. Fortunately I can change the voltage to 3.3V by changing the regulator part so I'll able to test in the coming days once receiving the new parts.
Could this 2.8V DVDD_I/O explain the missoperation of the SPI interface?
By the way, I wonder why this kind of part with an 1.8V main operation voltage is quite rigid in terms of the MCU interface voltage. It is quite common today systems with 2.8V or lower voltages.