CDCEL913 Output?

Hello,
I am trying to get a Clock Output of 26 MHz from an input of 12 MHz to a CDCEL913.
Without any changes to the registers I get a Clock Output of 12 MHz (i.e. the input just passes through).
I know that the CDCEL913 is responding to I2c commands, because 1) I can set and read back registers and 2) I changed the mode (I think it was the 04h register) and the output switched off completely.
So, looking at the documentation I though that I just had to change the settings of the 18h-1Bh registers to the correct values to get an output of 26 MHz (I don´t need a smooth change, so I don´t really need to switch between different PLL configurations). However, when I change the 18h-1Bh registers there is no change of the Clock Output.
Can anyone tell me what I have misunderstood and how to actually change the Clock Output?
/ Fredrik