I had similar problems when I wanted to deploy devices with newest software available.
I had to do a research on community.teltonika.lt and have found that there is a tool called modem_updater with which you can flash the modem firmware.
Just do:
opkg update
opkg install modem_updater
opkg install sshfs
modem_updater -g
Select version which you want to install modem_updater -v MODEM_VERSION_FROM_LIST
Looks like they pulling firmware from the server and mounting firmware folder with SSHFS. I was able to pull server address from modem_updater script - modemfota.teltonika-networks.com