Hello, thank you for steps! For sure I will try it, but probably next
month. Currently I do not have my board in hands, so I cannot do tests.
So I will just comment this part:

On Wednesday 01 January 2020 20:53:57 Florian Fainelli wrote:

- does it help if you go back to a kernel before and not including v5.1
which does not have commit 061f6a505ac33659eab007731c0f6374df39ab55
("net: dsa: Add ndo_vlan_rx_{add, kill}_vid implementation") or if you
can change your kernel, try something similar to [2] for mv88e6xxx and
see if it helps

[2]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e9bf96943b408e6c99dd13fb01cb907335787c61

Debian Buster has kernel version 4.19 and in backports is 5.3. I tested
both versions and there was no difference.

Was above mentioned commit propagated to stable trees (and there is
possibility that was backported to stable 4.19 version)?

I can use any kernel version since 4.17 (or 4.18?) as board is supported
by mainline kernel.

-- 
Pali Rohár
pali.rohar@gmail.com