You will Just have to be patient, atleast, that is all you can do since I personally see or feel that you don't have much of a choice, duelbits can decide not to help you with the claim that they do not have the private key to the network where you sent your coins to, and there will be absolutely nothing you can do about it.., the mistake is entirely yours and as unfortunate as it may sound, you are completely at the mercy of duelbits right now, they can decide to help you and they can decide not to help you..
So just keep reminding them and be patient, the issue is something they will help you solve at their own convenient time, I will only advise that you take the advise of AbuBhakar and put into consideration., find a duelbits representative who can help you be in charge of the case until it's resolved.
We hope that the team has access to the private key on the address he sent the coins to immediately resolve the problem, but he has realized that his mistake in transferring USDT to the TRC20 network was due to not carefully paying attention to the warning not to transfer unless the coins were already registered for deposit. However, we hope for the best solution to deal with that problem. I agree with your suggestion, you should continue to contact duelbit support to find a representative to handle the deposit address problem, but I am sure every report has been received and checked even though the team has not given a deadline to provide a solution to handle the problem that has been reported.