Frequently used the (Adafruit or els) DDS boards AD9850 with the wonderfull sketch of mr ....? (sorry)! To my surprise it's not only the frequentsy of the clock that has to be changed, but also another code to that board. (to start the correct working of the dividers ?). This is what made things work in my case. (Arduino Uno and the 9850 / 9851 boards.
AD9850 (part of the sketch)
int32_t freq = frequency * 4294967295/125000000; // note 125 MHz clock on 9850
tfr_byte(0x000); // Final control byte, all 0 for 9850 chip
And for the AD9851 it must be...
int32_t freq = frequency * 4294967296/180000000; // note 180 MHz clock on 9851
tfr_byte(0x001); // Final control byte, for 9851 chip