It does mention back USB-C port is the only Thunderbolt combined one. PDF pg 20 (marked page 6) Block Diagram imply DP (DisplayPort) is only on the rear USB-C port. This I can only assume your Thunderbolt adapter/splitter is not a compatible 'DisplayPort hub' & suggest looking at another one.
If you're using an older device that only has USB-A plugs, but your computer just has a USB-C connection, you can still utilize that USB 3.1 port with that device using an adapter that has the appropriate connections on both ends (USB Type-A on one end for the device and USB Type C on the other for connecting it to the computer).
Here's how: Plug the USB-C to HDMI cable into your Android tablet, using the normal USB-C charging port. Plug the other end of the cable into your regular HDMI cable. Plug the other end of the HDMI cable into your TV’s HDMI input port. Turn on your TV and make sure it's set to the correct input, where you'll find your Android tablet connected
FgopO.