As far as i know, there is no settings option to change the DNS server on cellular network, but you can do this using connmanctl. You may need to install connman-tools first.
To change the DNS run connmanctl in a terminal and enter services. This will show you a list of all network connections. Copy the service name of your cellular connection (starts with cellular_).To actually change the DNS, enter config <service> --nameservers <dns1> [<dns2>] [<dns3>].