Need to change BIOS settings for additional monitor support.

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From https://support.onlogic.com/documentation/tm_series/#tm610-mc610-50

By default, only the Mini DP ports in the expansion bay are enabled. The motherboard’s onboard HDMI and VGA ports can be enabled in the BIOS to support additional displays.

Press the Del and F2 keys at startup to access the BIOS.

Navigate to Advanced > Chipset Configuration. Set the following options:

Primary Graphics Adapter [Onboard]
IGPU Multi-Monitor [Enabled]
Active LVDS [Disabled]

Press F10 to Save & Exit.
The onboard HDMI/VGA are now used as the primary display ports and all ports are enabled.