tl;dr: If you’re looking for a functional UPS, avoid the Eaton 5E series. And if you want customer service, avoid Eaton.
In an attempt to make my network infrastructure more resilient, I decided to get an uninterruptible power supply, to keep the Wi-Fi on during power cuts. After some comparisons, I settled for the Eaton 5E 850i. It was a mistake.
UPDATE (2022-12-08): After a phone call, and another follow-up message, I finally got the support escalated to a product manager, who was quick to confirm that, indeed, the UPS was defective. It took another follow-up email to get a replacement actually sent to me. Fortunately, this time, it was detected.
The battery backup worked. However, I was never able to get it to be detected by the computer it was supposed to protect. While the 5E series is supported by NUT, it seems to be a well known issue that USB control is very flaky.
After 5 months of waiting for updates from the Eaton helpdesk, (and having to send nudges twice before getting any response), the only effective test I could do was to confirm that USB was also not detected with a Windows machine.
This week, we’ve had two power cuts, and the computer stayed on until the battery ran out, then just shut off uncleanly. Time to return the UPS and look for another brand.