I was asking about the address you have configured for the FreeRTOS+TCP demo application. If you are using static IP address, it will be set to the configIP_ADDR0-ADDR3 defined in the FreeRTOSConfig.h.
The simulated demo creates a virtual network connection with its own endpoint, so the IP address and the MAC address need to be unique on the network.
I just tested the latest mainline version of the posix demo and its working as expected for me, this demo is also hosted the Github CI of the FreeRTOS repository for pull request validation; you can take a look at the CI script that runs this demo on the CI here: FreeRTOS/.github/workflows/freertos_plus_demos.yml at main · FreeRTOS/FreeRTOS · GitHub
It uses a python script as an echo server that can be found here: CI-CD-Github-Actions/localhost-echo-server/local_echo_server.py at main · FreeRTOS/CI-CD-Github-Actions · GitHub