I mean you can connect it to a Uninterupptible Power Supply "USP".
It's not clear if they have a high power USB connector to connect USB batteries, however:
"Built-in battery backup for safe shutdown
On mode: 10 W
Standby: 0.4 W"
(https://thehelm.com/pages/technology)
> LTE backup. But then that's extra infrastructure against
> the promised simplicity.
There are low-cost routers with USB ports that allow plugging in an LTE stick.
> Also I don't see how encryption prevents the device from
> being physically destroyed.
You can create a backup and store it on an untrusted storage (cloud for instance).
I'm surprised these devices didn't take off 5 years ago already, as this is technology wise almost a step backwards into the 90s. But I think it's worth it, and in fact you don't need helm to set up a system like this yourself. Get a Raspi, stuff an LTE stick and a juicy USB storage into it together with external USB-battery in passthrough and you even have a superior device.
If you can accept the bad availability Helm looks like a fine solution to get your mail out of the cloud.
For example, in a power outage, Helm won't work. You say it's not a problem with a charged battery and LTE. I think you're talking about a cell phone and I don't understand how that's related. Sure, one could run Helm on a UPS with LTE backup. But then that's extra infrastructure against the promised simplicity.
Also I don't see how encryption prevents the device from being physically destroyed.