Generally the configuration of DNS resolvers doesn’t work as you want it to. It’s not “ask this one first and if it doesn’t know, ask another one”. It’s assumed that all are equal and have the same info. So mixing standard resolvers with others that know about some special local domain is not possible, because the public ones will never know about internal domain, they will tell the client that domain doesn’t exist and that’s it. But it’s a feature that people need, so normal resolvers allow to configure per-domain forwarding, i.e. that queries for some domains should be send to another upstream resolver.
Unfortunately, in this regard, RouterOS is not normal and this useful feature is not supported. There’s only L7 hack which has some limitations, but may be good enough for you.