Debian 10 (buster) is the last release to support QNAP TS-409 devices. These devices are not supported in Debian 11 (bullseye).
The reason is that the Linux kernel no longer fits into flash memory. These QNAP devices boot from a 8 MB flash chip. During the installation of Debian, we put the Linux kernel and a ramdisk (initramfs) into flash which in turn loads Debian from disk. Unfortunately, there's only 2 MB allocated for the kernel partition in flash, which is not enough to store a modern version of the Linux kernel.
Support for QNAP TS-409 was dropped from Debian 11 (bullseye) because of this limitation.
If you're running Debian on your QNAP TS-409 device, you have two options: