Vistara — Lost baggage

UA1 (SFO — SIN) on Dec 17 and then UK116 (SIN — DEL) on Dec 19.
4 bags were checked at SFO all the way to Delhi. All the 4 bags were delayed. Created a lost baggage claim with Vistara: DELUK14852. Vistara gave only 5000 Indian rupees for delay. Vistara delivered 3 bags on 21st December. One bag with tag number[protected] got lost. After 25 days, Vistara is giving 400 USD (20 USD/kg for 20 kg). Provided full list of items inside the bag and the full cost estimated as 1270 USD. Vistara keeps saying 400 USD saying airline policy, but could not give link to the policy. Also according to Montreal Convention, the compensation should be around 1370 USD. Not getting any closure after repeated emails and phone calls to Vistara. Please resolve.