If you are using a physical cartridge or a digital CIA of the base game, you cannot go online. The Nintendo 3DS eShop shut down in March 2023. The only way to obtain the 1.2 update is via a manually installed .cia file.
To update your Pokémon Ultra Moon game to version 12, simply follow these steps: pokemon ultra moon update 12 3ds world cia work
Pokémon Ultra Moon Version 1.2 update is a critical software patch released for the Nintendo 3DS on February 6, 2018. While primary Nintendo 3DS online services have officially ended as of April 2024, the update remains necessary for compatibility with various local features, ROM hacks, and ensuring general game stability. Version 1.2 Key Fixes If you are using a physical cartridge or