If you have your BIP32 Root Key you can import it in Bitcoin Core for instance with importdescriptors command:
importdescriptors
If it's that simple, he wouldn't have to open this thread.
He should've immediately saw his private keys in each of the default derivation paths of each Derivation path Tabs when he tested it IanColeman's BIP39 tool (
read his reply).
The main issue here is how MMGen generates its private keys from the mnemonic which isn't following the standard.
He just derived that "
BIP32 Root Key" in that BIP39 tool which is different from what MMGen does.
I can assure you from experience that creating a HD Wallet in Bitcoin Core from the BIP32 Root Key works with the method I explain...