You may install backup agent manually following the steps below:
1- Copy BackupAgent.msi file to the target DC in which you want to install the agent manually.
2- Run CMD as administrator.
3- Change your directory to the folder in which you copied the MSI file.
4- Run msiexec /i BackupAgent.msi install the backup agent.

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5- If you want to generate a log file during installation, then add the following parameters at the end:
/l*v name.log. For example:
msiexec /i BackupAgent.msi /l*v instal.log
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6- The installation wizard should pop up, just go through it until it completes.

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Once the agent installation completes, go to your RMAD console, right-click Backup Agents and select "Discover all backup agents instances".

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New backup agents should appear in the list.