I could not find documentation on using a 12-word BIP39 Mnemonic to restore a wallet through bitcoin-cli. It seems that importprivkey might be the right RPC call but when I provide the 12 words, it says "Invalid private key encoding" I also tried the following formats which were generated by Ian Coleman's page from https://github.com/iancoleman/bip39:
BIP32 Root Key
Account Extended Private Key
BIP32 Extended Private Key
Perhaps the master seed mnemonic is useless if all you have is bitcoin-cli and the bitcoind to which it's attached. Is that the case?