You can possibly make a data store that stores the current month when the leaderboard is updated, and when a new server is started it compared the saved month and current month.
And if its different then do the awarding function.
If you have your datastores set up so that each month uses a new DataStore completely, you can simply check the DataStore for the previous month and, if a player joins and is on it and has not been given the award already (you can put a boolean in their main data to track this), give them the reward.
One problem with this is your rewards would expire after one month because the game would stop checking the DataStores for any given month exactly 2 months after. I’m sure you could get creative with a way to fix this problem.
Edit: Didn’t realize someone answered but still will keep this here I guess