This is not specific to a Cellular dongle.

This is a side effect of improving support for SIM unlock. Sim lock information is provided in the network state, not the device state, so we need to create a network even when the device is disabled.

We don't run into this in Settings or the System Tray because we don't list networks when Cellular is disabled.

We can fix this in the OOBE UI by checking the Cellular Device state.

Right now mobile network in the UI is identified by mobile network name.

Do we need to remember this information if the dongle has been physically removed?
Are we sure that the next time the dongle is inserted, network ID will stay the same?
What if I have two SIM cards for the same mobile operator? What if I unplug the dongle, change the sim card and plug it again?
What if I change dongles, but the sim card will stay the same?

1. I'm not sure what you mean. The network name comes from the SIM. We do not cache this information. When the dongle is removed it should disappear (but see issue 780443).
2. We only support one Cellular network and it always has the same ID.
3. We do not cache any information in the UI so it should work as expected.
4. Ditto.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/97ede32758ee98a3ae035d070d57d94e535bd4ee

commit 97ede32758ee98a3ae035d070d57d94e535bd4ee
Author: Steven Bennetts <stevenjb@chromium.org>
Date: Tue Feb 13 19:32:15 2018

OOBE: Attempt to enable Cellular when showing the details dialog.

Chrome creates a default Cellular network when Cellular is available
but not enabled (or SIM locked). This CL does the following:
* Removes the extra (and incorrect) additional Cellular network in
  cr_network_select.js
* Attempts to enable Cellular when the Cellular detail dialog is shown.

This should cover the majority of Cellular configuration in OOBE.

Bug:   810328  
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ice8d7a58d3a9dba4784a74c11360ac04a1f9725d
Reviewed-on: https://chromium-review.googlesource.com/914595
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536424}
[modify] https://crrev.com/97ede32758ee98a3ae035d070d57d94e535bd4ee/chrome/browser/resources/chromeos/login/oobe_welcome.js
[modify] https://crrev.com/97ede32758ee98a3ae035d070d57d94e535bd4ee/chrome/browser/ui/webui/chromeos/login/network_dropdown_handler.cc
[modify] https://crrev.com/97ede32758ee98a3ae035d070d57d94e535bd4ee/ui/webui/resources/cr_elements/chromeos/network/cr_network_select.js

 Issue 811003  has been merged into this issue.