The STB expects untagged frames, so tagged interface lan1.103 will not work.
To configure tagged and untagged VLANs properly on DSA, you should use bridge-vlan configuration in LuCI, which was merged to master branch less than a week ago.
But the LuCI support is not backported to 21.02 yet, so you have to manually configure it in /etc/config/network, or switch to snapshot version,