Unlike other UART devices, only one pin of the TMC2226 is used for both the TX and RX connections. This means that for successful communication, the microcontroller/adapter needs to have a high impedance when expecting a reply from the TMC2226 as it needs to pull the UART line low. Since this is not possible for all microcontrollers the TMC2226-BOB has a series resistor on the TX line so the RX line can be pulled low regardless of what value TX is.
This means that the TX pin of your microcontroller needs to be connected to the UART_TX pin of the TMC2226-BOB and the RX pin of your microcontroller needs to be connected to the UART_RX pin of the TMC2226-BOB. Reversing this connection does not work and will result in failed communication.