The build process on ARMv8-M is somewhat tricky compared to ARMv7-M, so I recommend reading some of the articles provided by Arm or tool vendors to understand how to create your secure and non secure projects so that one can call into the other. I’m not sure which tools you are using, but some include built-in workflows that make it simpler.
Most use cases will run FreeRTOS on the non-secure side and non RTOS code on the secure side. For example: https://freertos.org/2020/07/security-for-arm-cortex-m-devices-with-freertos.html. To do that you must have configENABLE_TRUSTZONE set to 1. The hardware then enforces security by only allowing calls from non secure code to secure code through the secure gateway instruction - by design there is no software way around that so you need to see how your tools enable you to create those secure gateway entry points and place them in the correct place in memory.
The FreeRTOS download contains a few examples - which tools are you using?