Edit DNS server for DHCP

On Beryl, you’re presented with a Modify button then shown settings for various ISP connections. DHCP shows something like

Is there a way to override the DNS servers? I want to point to a DNS server accessible from my DMZ.

I don't know it this is correct what you are planning, but at first:
You are showing a screenshot of the 'Internet - Ethernet - Modify' Page. This is the WAN side. Else you are using DHCP on the WAN side and use the DNS configured by DHCP on the WAN DHCP router, or you set up the WAN manual.
Should your WAN direct to a local DNS server? Is this one configured with a correct forward server? I have doubt.

If you want to provide another DHCP in your LAN, go to 'Network - LAN - DHCP Server - Advanced - DNS Server 1/2' ... Here you can set up every DNS Server you want, that is provided by the DHCP answer from this router ... But make sure every device that gets a DHCP address is able to reach the DMZ.

There is no such menu on my GT1300. I’m running 4.3.17

There are not much information in your answer. You have no Menu on the left? You see no 'Network' in your menu? You have no 'LAN' under Network? You see no 'DHCP' in LAN?

It is possible, if your device is not in router mode, but in something like bridge.
But in that case, you need to modify the DHCP of the device that provides DHCP. That it nothing you can manage on your GL-iNet Router.

You can go to netowork->dns and set up your own DNS servers.

You cannot change these settings from DHCP. But you can try clone all these values to Static, if your IP do not change.