There is no “DLC unlocker” or “free NSP” that works without hacking your console – all such claims are scams.
To set up a complete feature set for on the Nintendo Switch (including the base game, updates, and DLC), you can utilize specialized software tools to merge these separate files into a single, convenient NSP or XCI file. Core Game Components Cassette Beasts ROM NSP UPDATE DLC - Switch Game
Once installed, return to the Switch home menu. The game icon should appear. Press + to check the version number (should show "2.0.0" with DLC listed under "Software Information"). There is no “DLC unlocker” or “free NSP”