Just to close this out:
The main problem was that:
From my location, one DNS Service 9.9.9.9 works about 5% of the time from my office, and never from my home. Oddly, their alternate 149.112.112.112 is very reliable, but slow.
Getting ahead of myself earlier, I already had SNORT running, and that was also blocking 1.1.1.1
So, rather than getting some redundancy by using 2 service providers, I was getting close to nothing.
I also had issues with the DNS resolver settings but finally got this sorted out, with everything running over 853 which was the objective there.
The only outstanding question I have is:
On main LAN, a PC (192.168.1.x) gets DNS server default IP of 192.168.1.1 via DHCP and an NSLOOKUP reports the server as "firewall.localdomain" as expected.
Replicating the setup on LAN 2, the PC (192.168.2.x) gets DNS server 192.168.2.1 via DHCP defaults but the NSLOOKUP reports the server as "unknown" - but everything seems to work - Admin WebGUI; servers on LAN; and Internet.
Changing the DHCP Server settings to hand out 192.168.1.1 as the DNS server for LAN2 resolves the issue and PCs on LAN2 show the DNS server name.
I thought the Interface IPs x.x.1.1 & x.x.2.1 as I have them, would behave the same - especially as the default action is to use these for each interface.
Is there an extra step needed for LAN2 to properly identify the DNS server?
Thanks