Whilst Asustor's ADM suits most people, it is possible to install ANY intel operating system on the nimbustor4/2 (as5202t/as5304t) as they run fairly generic intel hardware (asmedia sata controller, rtl8125 2.5gbe nic and ite system management chip)
in this gist I'll detail the steps needed to install and configure Debian11
Before proceeding you need to decide where you're going to install Debian.
Most people will choose to install it to a usb3 attached drive - either a usb3 ssd ( samsung t5 or similar ) or a usb3 hdd. This gives the most straightforward install ( just tell Debian to use the entire usb3 drive to install to ) and maximises available space on the NAS HDDs
The other option is to install Debian direct to the NAS hard drives - that means no extra drive hanging off the nas, and you can raid1 mirror the system partition across all drives for added resiliency ( though this is somewhat limited as the EFI boot partition is only on sda - so in the case of sda failing you'd need to boot off i
