So a few days ago I moved a DSL line over to eth0 on the Edgerouter Pro to serve as the WAN link. Previously a cable modem from Charter provided ethernet connectivity on eth0.
Before addressing what needs to be configured on the router I wanted to briefly summarize what typically happens when you’d like to use your router and firewall instead of that built-in to your modem.
Put the modem into bridge mode. This configures a DHCP server on the modem that will hand out addresses to any interface connected. Over ethernet, a pppoe client will be used to obtain an IP from your ISP. In this example eth0 was/is the ethernet WAN interface and pppoe0 is configured with our username and password to dial-into the DSL ISP.
Changes required on the Edgerouter Pro.
First, the Edgerouter Pro uses a zone based firewall and it occured to me that while the wan zone had interface eth0 assigned to it, the pppoe0 interface also needed to be assigned to my wan zone policy.
#set zone-policy zone wan interface pppoe0 #commit
Second, NAT was being performed on interface eth0 when it should reflect the fact that pppoe0 is now my gateway interface.
#set service nat rule 5010 description "Masquerade to WAN via pppoe0" #set service nat rule 5010 log enable #set service nat rule 5010 outbound-interface pppoe0 #set service nat rule 5010 type masquerade The specified configuration node already exists  #commit
Big thanks to the Ubiquiti forums member mrjester and Ubiquiti employee UBNT-Stig for their valuable contributions.