Hi. I have used the same tripod in the past for my Celestron 20x80 binoculars. I have now upgraded the tripod to the DPTR-880 PRO since the centre-column locking screw wouldn’t tighten hence rendering the centre-column unusable. I have experienced the same problem that you have mentioned with the DTR 550LW after dropping it accidentally. The altitude adjusting handle just rotates freely in place and doesn’t tighten at all. Upon closer inspection, I had found that a nut which holds the entire handle had been knocked out of place. After pushing it in place with a screwdriver, the altitude adjusting handle works fine.
This is the nut which got knocked out of place.
Hope this helps.