Note that “24 hour check-in” is not actually what you are looking for.
“24 hours check-in” just means that the reception is staffed 24 hours, so you can arrive at any time, including at 2 in the morning if you want/need to (but your stay must be booked from the day before in that case).
With or without “24 hour check-in”, you can always check-in at any time while reception is available. If the room is already available (usually because it wasn’t used the night before) they will usually let you have it right away. Otherwise they’ll be able to take your luggage and hold if for you until the room is available some time later (depending on the hotel they may bring your luggage in your room before you come back, or you’ll have to pick it up from reception or the bell desk when you come back).
What you are looking for is “early check-in”, where you get access to your room before the official check-in time. As described above if the room is available most hotels will give you your key right away. There must be a cut-off time before which they consider you should have booked the night before, but 9am is definitely after that time (or at least it was in most hotels I have visited).
If you want to be sure to have your room early, you have the book the night before.
Note that in some hotels even if your room is not available, in addition to holding your luggage until it is, they may give you access to facilities to take a shower or freshen up. It can be a dedicated “shared room” for this purpose, or it can be access to the gym, it really depends.