@rayray519 I am not familiar with Firewalla's products and checked their web site. Their Router Compatibility page lists the XB6 as "Not Compatible" (LIMIT-DHCP) but they also have a support page that offers some tips on how to get their products partially-working with the XB6.
If you really need to use the Firewalla Blue Plus device in your home network, your other option is to connect it to a compatible router and enable Bridge Mode on the XB6.
I know it's not possible with xb6, I'm not sure about xb7, more important to me is option to disable IPv6 than changing DNS, I'm using Hitron coda 4582 right now and there is option to disable IPv6 and change DNS...
Sorry. I assumed that you had just received a new XB7 gateway and were looking for a way to make these configuration changes.