It doesn't change anything...
But it looks that GUI is supported only for Espressif Thread Border Router that utilize Border Router board, not two boards with socs connected with wires - https://github.com/espressif/esp-thread-br/issues/20.

Hi, I'm playing with Espressif Thread Border Router example (ot_br + ot_rcp) and I've tried to add WEB_GUI for testing purposes. It looks that compilation option (CONFIG_OPENTHREAD_BR_START_SERVER) related to WEB_GUI and mentioned on following page: https://docs.espressif.com/projects/esp-thread-br/...