On the Client Machine

In the Client's “Remote Desktop Connection” Local Resources then click More

then check the [Ports] option click ok to save settings

On the Terminal Server Machine

Open Group Policy (to open the Group Policy Editor, go to run and type: gpedit.msc, In Computer Configuration, Administrative Templates, Windows Components, Terminal Services, Client/Server data redirection, double-click on Do not allow COM port redirection, and then click Disabled (to enable redirection).

Is there a way to check if my local COM ports are being redirected?

You can use the CHANGE PORT /QUERY command. Simply start a command prompt window and type the following command:

change port /query

You should see a similar output to the following:

AUX = \DosDevices\COM1

COM1 = \Device\RdpDrPort\;COM1:2\tsclient\COM1

COM2 = \Device\RdpDrPort\;COM2:2\tsclient\COM2

COM3 = \Device\RdpDrPort\;COM3:2\tsclient\COM3

LPT1 = \Device\RdpDrPort\;LPT1:2\tsclient\LPT1

PRN = \DosDevices\LPT1

As seen above all COM ports (1 to 3) are being redirected from session 2 (:2 after the COM port).

If the ports were not being redirected your output would be similar to this:

AUX = \DosDevices\COM1

COM1 = \Device\Serial0

COM2 = \Device\Serial1