Ok, thank you @admon.
Solution to my question based on the input of everyone (as of 2024-05-21):
- Hardware or Network Acceleration is a setting that's for the whole router.
- Disabling Hardware Acceleration possibly has little effect on the throughput but I have not compared this.
- With Hardware Acceleration disabled, the SQM works perfectly.
- So, if you need SQM on any wifi / interface / network, you have to disable Hardware Acceleration
Thank you.