Coinbase.com wallet or their their non-custodial wallet?

If you are using coinbase.com wallet, you don't own the private key and the only thing you can do is to pay accelerators. I don't how how legit the service you referred to is. Maybe another user can answer this question. 
If you are using the non-custodial application, you should be able to export the seed and import it into a wallet allowing you spend unconfirmed outputs.