With the factory default LPC11U35 firmware, the board appears in a PC as a mass storage device called CRP DISABLD. The file firmware.bin must be deleted and replaced by dap_cdc.bin (obtained from the repo CMSIS-DAP and CDC firmware for LPC11U35). After a reconnection, a new serial port must appears.
NOTE: we detected that this procedure (change firmware.bin by cdc_dap.bin) do not work on Linux machines, so try in a Windows.
If you want to change again the firmare, put a Jumper in JP3 (placed between the micro USB connectors).