The device has no agent of its own. A machine you already enrolled holds the SSH key and polls it. The device then shows in the fleet as its own row, marked via that machine.
Open Install agent. The box at the bottom is the short version. The working screen is on the proxy machine: Network › proxied devices, and only when that machine is online — the list is read from the machine, not stored as a form you can edit while it is away. Add a name, a host, a user and a kind (Teltonika RutOS, OpenWrt, Linux, FreeBSD / OPNsense / pfSense, a standalone ESXi host, or other unix). Press add to list, then push list and confirm. The push replaces the whole list. Copy that machine's public key onto the device before the first poll; the private half never leaves the proxy. The tab includes the install commands with the key already filled in. An ESXi key goes in /etc/ssh/keys-root/authorized_keys, then /sbin/auto-backup.sh, or the next reboot drops it. The device appears in the fleet marked via the proxy. See which machines have a shell is the report of who can run a command there.
Shell on those devices is a separate switch, proxy shell, set at install time. The hub cannot turn it on later. See which machines have it under Reports › Shell and proxy.