This issue has similarity with Hitbtc since they have problem too on Dogecoin but in there case was you cant withdraw Doge that you purchase inside the exchange since there doge rate is different to the global market price. They are luring user to purchase DOGE that cant be withdrawn and trap in the exchange alone. The same case took a lot of time or even now its still not resolved.
Mercatox is a mediocre exchange so expect a lengthy process just to fixed your issue. You are still lucky that there support is still answering. Try to use twitter to publish your story and tag them. This might help you to speed up recovering your asset.
Source to my Hitbtc story
https://bitcointalk.org/index.php?topic=5381570.0How long have you had this problem? Already solved? I don't know about you, but after all this is over or the 1st day arrives, I'm going to file a lawsuit with google asking for the withdrawal of these sites from their searches, as it is unacceptable that you can put money in and not withdraw it. Clearly they set these traps to deceive people.