I am managing a VSAT network that has two groups of users, using an RB750 as the main gateway router. It is configured with two interfaces each with DHCP, QoS and Hotspot applied.

My problem is that I don't fully understand the "hierarchy" in the DNS configuration. Consequently I can't get it to work with OpenDNS, which we want to use for filtering content.

Where do I enter the main DNS settings? In IP > DNS, or DHCP Server? other?

to make it simple, you could set openDNS to RB750 and set allow-remote-request=yes

on dhcp setting > network > make sure for each local network, assign local ip as the dns

all the clients on the two groups would then request dns to the local ip dan eventually use the opendns via rb750

Where do I enter the main DNS settings? In IP > DNS, or DHCP Server? other?

You enter it under the OpenDNS servers under '/ip dns'. All DNS requests from hotspot users (even to outside IPs) are captured and redirected to RouterOS's local DNS daemon. Insert OpenDNS servers under '/ip dns' to ensure these proxied requests go to your desired destination.

Many thanks, I will try that