With my 998, I used the XP 9 20 (no temp) transducer to mount in the hull, and the AS SIDB Y splitter cable. It works very well, at all speeds. You don't need the temp, as that is acquired through the hdsi ducer.
The XP 9 20 T should work as well. I assume you did use the splitter cable. If it isn't working, either the transducer is bad, or perhaps when you epoxied it in place, there were some air bubbles either in the epoxy or the hull itself. Hope you get it working.