Yes, it's probably the safest and fastest solution, I was trying to avoid the fees. Also this discussion might be helpfull to someone else in the same situation, especially if they don't have access to the app anymore for whatever reason.
Good for you to move it into another wallet asap, I am not sure what kind of mnemonic seed coinbox used. I tried importing into Iancoleman and got a different address (BTC, ETH, and LTC) also. You or others will be confused if the company closes the website/wallet because I didn't find how to access a private key on a phone app also.The private key was in setting>security but you're right, right now it disappeared, now you can back up the wallet on google drive (?).
I have a very little amount on this wallet and I tried to move it to Electrum wallet, but the cost for the transaction is higher than the amount itself, so either I leave it there or I don't know what else to do...