I think I'll mark this topic as closed. The rewiring seems to work and I am able to mirror traffic
solution is here: Swconfig versus etc/config/network
2 Questions remain open
1st Swconfig versus etc/config/network
its about how to have the righ UCI and Luci config for this setting
2nd [Solved] Router-set-up advertising
its about how to set up router advertising or enabling answer to arp request by this
configuration SOLVED was bad switch configuration file
Both question have their own topic open at the moment
Help is needed
Thanks to everyone