Problems with plugging Fios ONT into UPS

[mth0]

During power outages, my Fios ONT (GPON) used to shut itself off and had to be rebooted by the ISP's central bank. This was a nightmare during power outages--and we have a lot  of them where I am.  I have solar power with Tesla Powerwalls backup for my house, but the PWs take a few milliseconds to kick in, and this caused the ONT to shut itself down.  

To solve this, I bought a CyberPower ST325 , and plugged the ONT into it. It worked great for 4 years until it died of old age. I replaced it with a CyberPower ST425..

Unfortunately, after I did so, I started having brief but constant intermittent internet outages. For example, in 3 days there were 60 outages lasting 1-2 minutes each.  

An  ISP came out and saw that the wifi signal was too "hot", so he upgraded  the GPON ONT to an XPON ONT because (I think) it has greater bandwidth..  4 hours later, I started having multiple mini-outages again.😫

Another  ISP tech was dispatched to inspect and  when he saw the ONT was plugged into the Cyberpower UPS, he said that he'd seen that particular UPS cause multiple mini  outages similar to mine.

So, I unplugged the ONT from the UPS and plugged it directly into  the wall socket.

VOILA! No internet outages since.

I learned that pure sine wave UPS systems provide cleaner, smoother power output than simulated sine wave UPS systems. Pure sine wave output is identical to utility power and can safely run sensitive electronics like computers and medical equipment. In contrast, low end UPSs output a simulated sine wave with a stepped waveform that may cause issues for some devices—like my Fios ONT, apparently.

At this point, I still need a compact UPS that plugs into a wall outlet, that I can plug the ONT’s AC cable into, and that would allow the ONT to operate seamlessly, they way it should--without constant mini-outages.

The ONT does not draw a lot of power, and the UPS would only be providing power to the ONT for the milliseconds before the Tesla Powerwalls kick in. So I don’t need a maxed-out UPS, with USB inputs, etc. 

Any recommendations?

[mth0 · Accepted solution]

FINALLY!

I have an answer. I spoke with Tripp Lite Tech support (in advance of buying the aforementioned Tripp Lite wall mount UPS)

They said there are two reasons for ONT instability issues when plugged into the CyberPower ST425 UPS:

• When a converter is not loaded with enough wattage, it can become unstable on pass-through electricity. Whatever is plugged into the UPS should draw between 10%-20 of the specified wattage for the UPS. My ONT draws about 20 watts. The TrippLite 350U specified wattage is 180W, so that would work. The CyberPower ST450 was likely too much, and could have caused the instability, whereas the previous Cyberpower 350 I had was fine, until its non replaceable battery died. I bought the 450 to replace it because they stopped making the 350.
• Simulated sine wave (AC) can cause unstable pass-through because it is stair-stepped instead of smooth. The Tripp Lite Internet350U outputs pure sine wave (AC)

So, I just bought the Tripp Lite INTERNET350U, because that's what the Tripp Lite tech recommended, and it has a replaceable battery.

Most people have loads of devices plugged into their UPSs, hence the demand for higher wattage units. But I only need a UPS for the ONT, so not only did I not need 425W, it actually caused issues with the ONT's seamless operation--in addition to the simulated sine wave output, instead of pure sine wave output.