The solution is at this post http://forum.mikrotik.com/t/attempting-to-evolve-from-cavemans-failover/170048/59 thanks @jaclaz!
I tested with a real router and all seems to work: normal LAN, web browsing, Email and Tailscale.
The only thing that I’m not able to do is to connect to the ISP router (or another specific IP in the “outside” LAN) using a browser from the LAN.
This is not important but I’m curious if it’s possible to achieve also this!
Thanks
Nicola