AFAIK, there were at least 3 different ‘flavors’ of the RUT955 produced, during its lifecycle. You need to be sure, to fetch the matching image. I think, there were issues regarding openwrt on last 955-flavor.
To be shure, you should ask on Teltonikas support forum. Based on serial number (from sticker underneath), they can tell you the variant. I THINK, it also should be possible to derive variant from revision no., also on sticker. Search forum here, there was a report regarding issues with last “flavor” of RUT955.
Unless you have the last version of the RUT955, with latest RutOS, you can directly flash official openwrt from RutOS-GUI or, even safer, from CLI. NOT to save settings; may be, ‘'sysupgrade -F …’ required.
NOT to use …sysupgrade.bin, but …factory.bin . ONLY for very first flash, later on to use …sysupgrade.
Is there a reason you keep asking for instructions other than those provided in the official documentation?
And why you won't explain the current issue with following them?
By hitting the button to browse the local computer from the OEM or "original firmware" web GUI and browsing to select the factory.bin file from where you downloaded it from the official OpenWrt firmware website.
This will upload the file to the device - and is a similar process on most consumer routers.
SCP to /tmp always worked for me. You also need to do sysupgrade -F /tmp/firmware.bin then. May be, you log into the RUT955, cd /tmp, scp … , to fetch the file. firewall might interfere, otherwise.