As RutOS is based on OpenWRT, the default (and the only) package installer available on these devices is opkg. Before installing any packages run opkg update. Some repositories will fail to update - this is okay.
Afterward, you can install the packages using opkg install <package_name> command.
However, the package modbus-cli does not seem to be available on opkg. I'd recommend exploring this comment, perhaps the provided commands will suit your needs, as they are preinstalled on the device.