In the eShop of the account you bought the DLC on:
- Check if primary console: Under the first menu when you enter the user settings, scroll down, there should be an option "Primary Console" that says the currently used console is the primary console
- NSO + Expansion Pack: Also in the user settings, under Nintendo Switch Online - you should have your subscription entered here (although I only have the normal sub, so I'm just guessing it would mention the expansion pack here)
- Look up the DLC itself in the eShop, and make sure it says Purchased. Otherwise, it might give you the option to redownload it, if it somehow just got deleted.