@ulmwind the documentation you had mentioned is exactly the one I am following, but I am facing the issues I described.
@lleachii can you elaborate your answer with a working example of /etc/config/network? The examples from the documentation are not working for me, and I am sure the VLAN ID is correct.
@lleachii I see you did not declare the devices nor the vid. Are they missing?
It seems in one of my tests I could get it to work, but it was using IPv6 on the VLAN device I had declared. Once I added option ipv6 '0' it was not working. Is IPv6 mandatory for to get the VLAN work internally? I have dhcpv6 disabled on the downstream device, so this IP is not coming from there.