Since you’ve already copied the file to TRB255, I’ll simply guide you on extracting all .ipk files from snmp.tar.gz and installing them:
Firstly, navigate to your chosen directory. I recommend using the /tmp/new directory.
In the directory with your file, execute this command: tar -xzvf snmp.tar.gz.
Then use the command opkg-call --offline-install <your_directory>.
With this, SNMP should be installed without needing an internet connection. SNMP might not show in WebUI at first. But after device reboot it will be accessible.