My understanding is that you need to submit a claim detailing what was in the bag and the current value of each item (including the bag itself). You can then state you are making a claim to the value of X under the terms of the relevant clause of the Montreal Convention. They aren’t going to just hand over a cheque for the cash equivalent of the maximum SDRs without all that.
I would send that through this portal in the first instance:
https://www.qatarairways.com/en-th/help/contact-us.html
Select "Complaint", related to "Baggage services"
Be calm, detail only the facts, include any previous correspondence, flight and bag tag numbers etc, along with your itemised claim. And give them a reasonable deadline by which to respond (a fixed date, say 28 days later). Bear in the mind that the person reading your message won't know anything about it, won't be responsible for what has happened so far, will have to undertake internal checks, and will then just be assessing the eligibility for a claim based on the facts. So try not to be emotional or critical, those won't help.
If they do not respond you basically have 3 practical options - chase them again, or refer it to one of the many online claims companies (who will take a percentage of any settlement), or make a claim via the DOT in the US. Trying to launch a legal action in Qatar will be an expensive futility, probably costing more than you have lost.
Good luck!