Is output of lsusb showing your dongle as mass storage device ?
According to the article which is shared by you, the solution consisted of few packages, importantly usb_modeswitch.
In one of the conversation on Ubuntu forum, the idea was to reinstall this package.
The latest version is usb_modeswitch 2.6.1-2 . If you install(re) and reboot the device then it should be solved.
Then why not unmount it first via file manager, then follow those steps given in your link ?
In Arch Linux Wiki, I found this :
ModemManager does not recognize the modem
In case ModemManager does not recognize the modem, check the unit status of ModemManager.service. If you get error messages such as Couldn't check support for device and not supported by any plugin, you may have to whitelist your device using the ModemManager filter rules.