Found my answer in here: https://community.tp-link.com/en/home/forum/topic/666206 Thanks to @Jasper97, this switch does not support IEEE 1905.1 which is required for backhaul to work.

Hello, I have a system set up with 5 Deco devices, configured in Router Mode. 4 Deco are X60, with Firmware 1.2.5 Build 20240106 Rel. 2319 1 Deco is X20, with Firmware 1.1.8 Build 20240119 Rel. 37701...