As established some pages back (e.g.
here
and
here
- again with an e1000e Intel NIC) this problem is not specific to Realtek NICs. And "new" kernels (as in 5.10) does
not always work with Intel NICs.
The issue seems to have something to do with the way the Debian kernel is configured, or with some Debian-specific kernel patches, and not with a specific NIC manufacturer. While the problem might go away with a different NIC, no-one has of yet established the exact cause for the problem so changing NICs will be hit-and-miss. What does seem to work is building your own vanilla+rt-patched kernel.
If someone had time to build their own kernel from Debian sources, that would be an interesting test to narrow the cause down further.
