UPS lost our entire Christmas shipment

UPS lost our entire Christmas shipment consisting of 10 large boxes with 400 units of products. The value of our products are $8,000 with a selling price total of $15,000. The boxes’ last scan is in a UPS facility then it’s been dormant ever since. We use the UPS partnered account for the discounts but because of this, UPS won’t investigate or open a claim. As for Amazon, they say I must wait 4 months and 3 weeks for the eligibility of an investigation to happen. Basically just to reconcile the units which weren’t received. Has anyone had experience with this? It’s insane that either company refuses to acknowledge or take responsibility. This is so much money to lose and we know Amazon will never reimburse us 4-5 months from now either.