Have you taken apart the charger yet?. If its a wallwart then if you carefully run a fine hacksaw around the groove you can open up the moulding. One common fault if its of this sort is the thermal fuse on the mains transformer breaks. To check this see if the primary of the transformer appears to be open circuit. If on inspection, the transformer looks alright, i.e. no signs of burning or overheating etc. then very carefully pull the tape of the transformer and you will find a little thermal fuse which is open circuit. Solder a short across it. Re-assemble the case and tightly wrap insulating tape around it to hold the two halves together. Do not operate the "repaired" unit unattended, because it has no thermal protection on it and it might catch fire.
To get back to your question, have have 24V, you need 21V at .3A, so you need to drop 3 V at .3A, so you need 3/,3 = 10 ohms, at a power of 3 X .3 = .9W, i.e.1W.
Frank