Importing the private key returns 'null'
"null" means that the import worked... Your wallet should now contain the address/private key that you imported. Otherwise it would have returned an error message and an error code.
To get the transaction history for this address/private key, you may need to restart Bitcoin Core with the "-rescan" option, so that it rescans the blockchain for transactions relating to that address.
I'm currently looking for a way to generate a private key linked to the address using my 12 words (BIP39?)
The BIP39 Mnemonic Code Converter can do that...
https://iancoleman.io/bip39/ I'd recommend you download the page (right click -> save as...) and then run it on an offline computer or using a live CD/USB with the networking disabled to reduce the chances of exposing your seed to hackers/thieves.
The tool itself is pretty easy to use, you just need to make sure you have the correct "Derivation Path"... which can be different in some wallets. Which wallet did the 12 words come from?