“A sag or swell phase to phase will force battery operation then your load drops. Battery is dead and UPS turns off.”
He says his battery works fine when power is lost.
I’m not sure what turns off that UPS.
Input can swing +/- 10%.
Those little systems also have outputs that are not battery protected. They are labeled.
Sounds like a faulty UPS. I doubt an ability to log into it and read what is happening.
There might be some minimal info from the display if it has a display you can cycle through.