Yes, it has almost certainly reverted to default settings. The reason upgrade from server offers FW version RUT9XX_R_00.04.75 even though it is not the latest is because there was a known bug with the keep settings feature after Teltonika changed the firmware versioning system. So for versions older than RUT9XX_R_00.04.75 it is necessary to upgrade to version RUT9XX_R_00.04.75 in order to keep all custom settings after the upgrade. Since version RUT9XX_R_00.04.75 and all later versions you can upgrade with keep settings without any worries. Because you upgraded from RUT9XX_R_00.03.832 directly to RUT9XX_R_00.06.00.2, the router's settings must have reverted to their default state due to the issue I described.
Now since a SIM card is already inserted in the router, the router should have Internet access even with default settings, unless your Internet service provider (ISP) requires additional settings in order to achieve a connection. For example, it may require PAP/CHAP authentication or APN. In that case, I would suggest contacting someone who is on site to configure any required mobile settings in order to achieve a connection and you should be able to regain remote access.
One more thing: there have been a lot of changes since version RUT9XX_R_00.03.832. One important change that you may not be aware of is that it is now mandatory to change the router's default password from admin01 to a password than contains at least 8 symbols, at least one upper case letter and at least one digit (for example, Abcdefg1). So if you decide to contact someone to configure the router, you should probably think of a password and inform them of this, since you can't configure anything before you change the password.