An update for anyone following, or landing, on this discussion.
It turns out the AT&T router will not work for the application needed. For that reason it has been switched to IP Passthrough
mode and the Edgerouter is again the gateway device and DHCP client for its WAN address.
So, I, once again, began searching for a means to get the Edgerouter to do the Sonic Dynamic DNS updates. And, I think I have found the hook. It is described in this forum post
on the Ubiquiti forums. Essentially, after each DHCP update, EdgeOS looks in a specific location for exit scripts to execute. All that is needed is to place an appropriate script in that location. And, in fact, the post's author even provides a sample script.
With this information in hand, I then googled for "edgerouter dynamic-dns dhclient-exit-hooks" and got a lot of hits with other, similar, scripts to do the same thing.
Still untested. But, it looks good.
- As Kelsey previously warned. Scripts do not persist after an upgrade. You have to re-install them.
- You still need a simple script somewhere, could be on the router, to get the authentication token.