I would try to fill the low areas with a modified thinset prior to the Ditra installation. The surface needs to be really flat before installing the Ditra. This may require a couple of skim coats to get a fairly smooth surface.

This project was on hold and is now back. The floor was prepped as discussed above and all low areas were filled with a modified thinset to bring flat. I am ready to lay the Ditra and my current dilemma is what type of thinset to use now that the floor is about 80% covered with modified. There were more uneven surfaces than first anticipated and it required much more work to achieve a flat surface. Do I no longer have a plywood application as far as Ditra is concerned? Thanks in advance.

You can use modified to install the Ditra.