It's quite a while since I did mine. There is one important change to make to the livebox settings, but I can not what it is. I will take a look at my settings when I get a chance. It is not in the DMZ settings.
I have not mastered the IPV 6. It leaks when turned on at the livebox, so I just use IPV 4.
My setup is adsl. I don't know if fiber will be different.
My secondary router is just that. It is a TP link. It can not be used as a primary router and doesn't have a WAN port .
I have just ditched Sky and bought a Freesat box. I can access Bbc iPlayer, ITVX, My 5 and Netflix without problem.
I use express VPN and have done for several years. I have loaded all the different UK locations to backup files for open VPN. I can then easily change the location my router gives out, should one of the locations go down. I am currently using "Wembley" .
Hope this helps.
Richard