I assume you're accessing it over Wi-Fi?
In which case, that's just Nickel aggressively dropping Wi-Fi to conserve battery.
There's a hidden dev setting (that can be toggled via NM) to keep WI-Fi on *forever* (and I mean that: the device won't suspend, and will eat through a battery in no time if unplugged).
Otherwise, my usual quick hack is to keep the Wi-Fi popup open: that'll prevent Nickel from dropping the connection.
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The better solution to that conundrum is, of course, SSH over USB via USBNet
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