Please add a screenshot of your current switch configuration page (under 19.07 or other pre-DSA / swconfig image) in Luci, and I will try to help reproduce the same setup under DSA. I know the definition is also under the switch parts of the config file, but it's difficult to see exactly which ports are available, and how they are setup. The graphical overview of the switch page is much nicer IMO
The concept of DSA is to make it seem to the user that the ports work independently, even though there is a hardware switch.
Each port has a unique name like 'wan', 'lan1', 'lan2' etc. These are supposed to match the manufacturer's name marked on the case but may not always.
Including a port name in a network (list ifname 'lan1') causes the port to send / receive untagged packets. Including a port name with a VLAN number (list ifname 'lan1.100') causes the port to send / receive tagged packets.
So to send tagged packets for the two networks to the SG108E switch would be lan4.1 in the lan config, and lan4.100 in the TV bridge.
config interface 'tvbridge'
option type 'bridge'
option proto 'none'
list ifname 'lan1'
list ifname 'lan4.100'
Here you're receiving untagged packets from the ONT on port lan1 then tagging them 100 to send out on the trunk cable from lan4.
The Livebox ... cannot work in Bridge.
There is therefore no need to talk about ONT in the configuration. All the ports of the LIvebox send the normal stream for the Internet as well as the TV streams
In my case I bypass the router functionnality for the TV which will then be redirected correctly to the TV box by the SG108E
I already had a lot of trouble getting this configuration to work ... so if now the DSA is added
Again, can you please screenshot the switch page in Luci? This contains the same information as the network file already does in the first post. It's a bit difficult to see how the ports are laid out on the switch by looking at the config file. The switch page displays this much more clearly.
I assume port 1 is the Livebox and port 2 is the SG-108E.
So the Livebox is using tagged packets to differentiate TV from Internet.
In that case you would only need one cable from the Livebox to your router, and it would access both VLANs with tags.
This defines a software bridge with only one member, and no protocol. Thus, it does nothing useful. The TV traffic is being switched by hardware in the swconfig.