A few refinements:

1. If there is pushback from a third-party such as one of the so-called OTA's, call the carrier. Where they hold the funds, many are simply processing the refund and doing an ADM to the OTA for its commission / fee. A lot easier than dealing with chargebacks and other passenger dissatisfaction. This will generally require a supervisor / manager. But, what was unheard of a few weeks ago, is now common.

2. In either case, simply file a chargeback and provide:
a. E-ticket receipt showing full routing, e-ticket number, and total charge.
b. Proof that the air carrier cancelled. Could be an email or a screenshot of a web page.
c. Your request for a refund.
d. Either denial of refund or a note from you to the effect that the

3. DOT does not accept complaints against TA's. It does not regulate them and its Title 49 authority is expressly limited for this purpose to commercial air carriers. Your complaint may well be forwarded to some carrier on your e-ticket for a response, but if the carrier has chosen not to handle as per #1 , it will simply respond that it is not the responsible parrty here.

In any event, the chargeback is critical as DOT may punish a carrier, but that does little for the passenger. A properly documented chargeback may well result in a temporary credit within a day and a permanent credit in short order. By documenting properly, you make it such that the whoever holds the funds will not be able to effectively pushback.