PinePhone modem keeps "disappearing"

[Kevin Kofler]

I am already using ATTR{power/control}="on" (since January 18, see zetabeta's snippet). It does not solve the issue. Often, the modem does not even come up on USB to begin with, so the udev rules will definitely have no effect, and even when it does, the udev rules do not prevent it from disappearing from USB. The modem is still running, mind you (as it can be talked to over ttyS2), just not on USB.