Workaround / Solution found by changing the option to my mezos service configured to accept
0.0.0.0:29091 port (i.e. accept connections from any network on 29091 port).
Thanks a lot @tommoulard for your comment, @dduportal for the previously mentioned post that helped me solve this and P.W, for the suggestion of using 0.0.0.0 on mezos.
Note 
I'm still wondering why there is no better networking / what is wrong on the networking level and why it was unable to establish properly communication by specifying the hostname relative to the shared network.
I tried adding container_name: and hostname:, but none of those options worked.
I guess it's probably more a docker issue than a traefik issue, though.