Reproduce the issue with the stock firmware, then provide a detailled report to Asus in the official Asuswrt forum. I must admit I'm unable to understand the problem you are trying to describe, and I know there are quite a few users who are succesfully using Dual WAN.
Aug 1 08:01:13 rc_service: wanduck 422:notify_rc restart_wan_if 0
Aug 1 08:01:13 dnsmasq[444]: read /etc/hosts - 5 addresses
Aug 1 08:01:13 pppd[514]: Unable to complete PPPoE Discovery
Aug 1 08:01:13 pppd[514]: Hangup (SIGHUP)
Aug 1 08:01:13 pppd[514]: Terminating on signal 15
Aug 1 08:01:13 pppd[514]: Exit.
Aug 1 08:01:13 kernel: Attempt to kill tasklet from interrupt
Aug 1 08:01:13 kernel: br0: port 1(vlan1) entering forwarding state
Aug 1 08:01:13 kernel: device eth0 left promiscuous mode
Aug 1 08:01:13 kernel: device eth0 entered promiscuous mode
Aug 1 08:01:13 kernel: br0: topology change detected, propagating
Aug 1 08:01:13 kernel: br0: port 1(vlan1) entering forwarding state
Aug 1 08:01:13 kernel: br0: port 1(vlan1) entering forwarding state
Aug 1 08:01:15 wan: mac clone: [wan0_hwaddr] == [00:24:**.**.**.**]
Aug 1 08:01:15 pppd[689]: Plugin rp-pppoe.so loaded.
Aug 1 08:01:15 pppd[689]: RP-PPPoE plugin version 3.11 compiled against pppd 2.4.7
Aug 1 08:01:15 pppd[691]: pppd 2.4.7 started by shaobing, uid 0
Aug 1 08:01:16 rc_service: wanduck 422:notify_rc restart_wan_if 1
Aug 1 08:01:18 wan: [wan1_hwaddr] == [AC:9E:17:93:C0:C7]
Aug 1 08:01:35 rc_service: zcip 697:notify_rc start_firewall
Aug 1 08:01:35 dnsmasq[444]: read /etc/hosts - 5 addresses
Aug 1 08:01:35 zcip client: configured 169.254.44.74
Aug 1 08:01:35 start_nat_rules: apply the nat_rules(/tmp/nat_rules___)!
Aug 1 08:01:50 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:03:05 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:04:20 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:05:35 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:06:51 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:07:50 rc_service: httpd 450:notify_rc restart_wan_if 1
Aug 1 08:07:52 wan: [wan1_hwaddr] == [AC:9E:17:93:C0:C7]
Aug 1 08:08:06 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:09:21 pppd[691]: Timeout waiting for PADO packets
Aug 1 08:10:36 pppd[691]: Timeout waiting for PADO packets
Fiber Modem is connected, you can connect to the network using 1WAN, everything is normal / when using 2WAN, WAN can not get to a public IP, lan (when WAN2 used) can not get into the fiber port Modem intranet IP. That is the time when using 2WANYou can not detect PPPOE link.