Upgrading firmware via ssh

I have a RUT955 router which will not recognise my Virgin Media SIM - a SIM which I use all the time without problems in my phone. It has been suggested on the Teltonika forum that a firmware upgrade may sort out the problem, so I tried upgrading from RUT9_R_00.07.00 to the latest firware
but I couldn't get this to work.

Can anyone tell me if/how I could perform this upgrade via ssh?

Apparently the latest upgrade also installs OpenWrt v21.02.

I'm not sure how to display the current firmware from a shell. I thought it might be in /etc/os-release but it isn't.

If you are looking to install the official OpenWrt 21.02 firmware on your device, you can find information here about how to do it. However, if you are trying to use the firmware you linked in your OP, you should talk to the maintainers of that version (Teltonika) since it is a customized release (not the standard OpenWrt releases that are supported here).