Last time I had only my mnemonic key and when the coin was no longer supported by the wallet i had to find a private key to get access which I did not write down because I thought mnemonic was enough. I was never able to retrieve those coins after that even though i had the mnemonic and spoke to the tech team. That was with the Guarda Wallet. I think you need both private key and mnemonic as a backup. And this option should be in the wallet interface like every other wallet has. should not have to fish around for it.