Contact Qatar one more time with a "notice before action" explaining that your claim for damage your luggage as per the Montreal convention has not been resolved and setting out what they need to do to remedy within (say) 2 weeks, keep it all reasonable factual and focused, no long messages or emotions needed. Ensure that you understand your rights under the Montreal convention.

If they fail to remedy then issue a small claims court process against the Qatar Airlines legal entity in your country (assuming they fly to the country in which you are ordinarily resident).