When you apply time remapping, 2 keyframes will be added to the start and end of the original clip.
So, if your clip is trimmed (you set in-out points manually) then you will not see those keyframes because, as mentioned, they are at the start and end of the original duration. You can use the add keyframe button (highlighted in the picture) to add keyframes manually at the in-out points of the trimmed clip
If you have a trimmed clip the easiest approach is to Pre-compose moving all attributes to the new comp and trimming the new comp to the length of the layer(s). You do not need to open the pre-comp.
Then apply Time remapping to the entire clip.
A workaround would be to move the CTI to the in point (i) and add a keyframe for time remapping using the little diamond icon in the middle of the keyframe navigator, and then go to the out point (o) and do it again. Now you can start dragging time around.