Hi Erik, thanks for contacting support. I would first make sure that the module can be inserted from the hostOS as in your second comment above.
The “Unknown symbol in module” usually indicates that the kernel source you have compiled against does not match the running kernel in the device.
Please check that you have modified the VERSION argument in the Dockerfile from kernel-module-build to match the BalenaOS release the target device is running.
ENV VERSION '2.29.0+rev1.prod'
2.29.0+rev1.prod should match the release on your device.
Once you have rebuilt the module, see if the insmod in the hostOS works, and retry the insmod from the container, although the latter might be a different problem altogether.