Hello.
I'm starting to work with my RB800. I'm trying to change my providers DNS for others, for example Google 8.8.8.8. Any time I changed them in /ip DNS the internet stop working. I also add this firewall rule to force the use of Google DNS
/ip firewall nat
add chain=dstnat protocol=udp dst-port=53 action=redirect
add chain=dstnat protocol=tcp dst-port=53 action=redirect

But without success. Any idea about what I'm doing bad? I'm new with this kind of systems. Thanks!

Your provider is blocking other dns connections for safety reasons probably. Ask him for explanation or bypass the block by tunnel to a place you can use your desired dns server.

Thank you for your answer. I have contacted my provider. They said me to leave the /ip DNS servers and to change the details of the /ip DHCP-servers networks. I change the DNS of the LAN network of my system, and the internet is going. Anyway I'm not sure the configuration is working.
In fact I wanted to add OpenDNS DNS. Now when I put them DNS the system works, but the OpenDNS service in fact it is not working. In the site of OpenDNS is writen that if the provider use a Proxy server his system in not going to work. How can I know if my provider use a proxy server?
In some sites (for example http://www.phy2vir.com/configuring-mikr ... e-opendns/) I have seen that they change the /ip dhcp-client set use-peer-dns=no option to force the use of the locals DNS, but in my router I don't have any network at /ip dhcp-client. Is this normal?
Thank you very much for your time.

It's actually again better to get in touch with the isp to ask him about your possibilities in his network. Seems to be willing to cooperate with you. You don't need to adopt dns servers from incoming DHCP message,you can use manually entered values. Check manual for DHCP client and ip dns settings.