WAX204 cannot run on UPS

I have a WAX204 which I need to plug into a UPS. It runs okay if the UPS is plugged into AC power. However, whenever the power goes out, the router reboots and gets stuck in a boot loop. The loop happens whenever it is about to turn on the WiFi light, then it reboots back to the red light and repeats all over. However, once the AC power is restored to the UPS, everything is back to normal.

I had thought it was the UPS power, and I bought a 2000VA / 1200W power supply, which is the same. How can I use this router on UPS?

Here is the exact UPS: https://www.hikvision.com/africa/products/premises-distribution-system/ups/back-up-type-ups/ds-ups2000/