How to restore wallet from 15-word seed mnemonic?
Mnemonic phrase from 2015 or earlier, and not an Electrum seed?
Then there is a high probability it is blockchain.info's legacy wallet recovery mnemonic.
Which can be recovered here https://blockchain.info/wallet/forgot-password
Correction:
This is a blockchain.info mnemonic. It contains your WALLET ID and Password (pre 2013 contains only Password and/or password plus alias)
You can decrypt the mnemonic using blockchain.com webpage they have a legacy (HD is only 12 words) page which changes from time to time.
With that you can login to your account and access your coins.