What are RS232C straight and null modem cables and which one do I use for the TB-SLR-RS2?

Use a straight RS232C cable when connecting to the TB-SLR-RS2 and not a null modem cable type.

A null modem type cable allows 2 DTE devices to connect directly without the modem interface (hence the term "null modem"). To achieve this the cable swaps the send and receive paths so that the transmit always corresponds to the receive on the second device.

In the case of the TB-SLR-RS2, it is a modem that converts electrical signals to radio waves and back again by a second TB-SLR-RS2 or equivalent. Therefore it requires a straight cable when connecting to the PC.