Modem Manager is a service that provides support for USB Modems to run on all major Linux Distributions. If Modem mangers is fail to run the services you modem does not work any more it provides the support for USB Dongles and modems.

It has bug that the modem manager is services are not started in systemd system and service managers. Modem Manager support the 2G, 3G, 4G and 5G devices under the hood. This service is the vital part needed to best modem connectivity as per latest trends. Some EL have disable this option so you will enable this option manually. Please follow all the steps for fixing this issues.

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Prerequisites

Step-I (Check whether USB Modem / Dongle is detected by the Linux Kernel)

Firstly Plug-in your USB Modem after that type the following command on Linux Terminal

# dmesg | grep -i usb
Step-II (Modem Manger Packages is installed or not)

Secondly check the package is installed

# rpm -q ModemManager
 ModemManager-1.4.6-1.fc22.x86_64

OR

# dnf info ModemManager

After that check the Sample Output

Installed Packages
Name : ModemManager
Arch : x86_64
Epoch : 0
Version : 1.4.6
Release : 1.fc22
Size : 2.4 M
Repo : @System
Summary : Mobile broadband modem management service
URL : https://wiki.gnome.org/Projects/NetworkManager
License : GPLv2+
Description : The ModemManager service manages WWAN modems and provides a consistent API for
: interacting with these devices to client applications.
Step-III (Install the ModemManager Services if not installed)

Then install the missing package

# dnf install ModemManager

How to fix this ?

Step-I (Start the ModemManager Service)

First of all start the Modem manager services as soon as possible.

# systemctl start ModemManager
Step-II (Add ModemManager to Systemd Service and System Manager)

Secondly enable the services at boot time so that it will survive.

# systemctl enable ModemManager

Restart / Reboot the PC after that Plug in the USB Modem / Dongles .

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Congratulation ow you have fixed the modem manager is not working on CentOS and RHEL.