@johjames
I'm not familiar with either the "Firewalla Purple" or the "Asus RT-AC88U" but I note from the Firewalla documentation that it supports IPv6 on both the WAN and LAN side.
If the setup works with the Firewalla Purple but not without, I strongly suspect it is IPv6 related (my understanding is that Community Fibre need the device connected to their ONT to support IPv6).
I've had a quick look at the Asus RT-AC88U manual:
https://dlcdnets.asus.com/pub/ASUS/wireless/RT-AC88U/E10302_RT_AC88U_Manual.pdf?model=RT-AC88U
Section 4.4 (page 82) talks about IPv6 and has 4 steps listed.
"Connection type" selection seems the most relevant but sadly doesn't list the choices.
My Draytek (in the similar setup section) has:
Connection Type:
DHCPv6 Client
WAN Connection Detection:
Always On
Then I have two options unchecked (RIPng Protocol & Bridge Mode).
The Asus manual shows this in the IPv6 setup:
View attachment 5569
What options do you have in "Connection Type"?
Asus' generic support suggests that "Native" might be an option, I would give that a go. If it then offers DHCP try that
(edit).
I might start with Router Advertisement off and see how that goes (from my extremely limited understanding that is intended to help IPv6 work on the WAN side which isn't the current objective).
Hope this helps!