mmcli: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/mmcli - from package: ModemManager

You may install the required package with command apt-get install {package}

Error in other languages:
mmcli: bevel nie gevind nie
mmcli: comandă negăsită
mmcli: comando não encontrado
mmcli: commande introuvable
mmcli: command not found
mmcli: käsku ei ole
mmcli: không tìm thấy lệnh
mmcli: komanda nerasta
mmcli: Kommando nicht gefunden.
mmcli: kommandot finns inte
mmcli: komut yok
mmcli: nie znaleziono polecenia
mmcli: níor aimsíodh an t-ordú
mmcli: no se encontró la orden
mmcli: no s'ha trobat l'ordre
mmcli: opdracht niet gevonden
mmcli: parancs nem található
mmcli: perintah tidak ditemukan
mmcli: príkaz nenájdený
mmcli: příkaz nenalezen
mmcli: команда не найдена
mmcli: командата не е открита
mmcli: 命令找不到
mmcli: コマンドが見つかりません