Issue with PPPoE Connection as Secondary WAN

[dnboe]

Hi, I had to create an account just to ask this question because I've been struggling with this issue for days now.

I have two internet providers: one with automatic IP connection and the other with PPPoE connection. Both work without any issues, but now I want to use them in Dual WAN mode on my ASUS RT-AX88U Pro router.
When I set the PPPoE provider as the primary WAN and the automatic IP one as the secondary, everything works fine (primary WAN connected and secondary WAN in standby). However, when I reverse the setup, making the automatic IP provider the primary WAN and the PPPoE provider the secondary, I get the error below on the secondary WAN:

Any idea on how to solve this issue? Thank you in advance!

 

[dnboe]

System log keeps showing this:

Oct 28 21:37:34 pppd[5020]: Connected to 5c:83:82:c8:2e:c5 via interface eth0
Oct 28 21:37:34 pppd[5020]: Connect: ppp1 <--> eth0
Oct 28 21:37:35 pppd[5020]: PAP authentication failed
Oct 28 21:37:35 pppd[5020]: Connection terminated.
Oct 28 21:37:35 pppd[5020]: Failed to disconnect PPPoE socket: 114 Operation already in progress
Oct 28 21:37:45 pppd[5020]: Connected to 5c:83:82:c8:2e:c5 via interface eth0
Oct 28 21:37:45 pppd[5020]: Connect: ppp1 <--> eth0
Oct 28 21:37:46 pppd[5020]: PAP authentication failed
Oct 28 21:37:46 pppd[5020]: Connection terminated.
Oct 28 21:37:46 pppd[5020]: Failed to disconnect PPPoE socket: 114 Operation already in progress
Oct 28 21:37:56 pppd[5020]: Connected to 5c:83:82:c8:2e:c5 via interface eth0
Oct 28 21:37:56 pppd[5020]: Connect: ppp1 <--> eth0
Oct 28 21:37:57 pppd[5020]: PAP authentication failed
Oct 28 21:37:57 pppd[5020]: Connection terminated.
Oct 28 21:37:57 pppd[5020]: Failed to disconnect PPPoE socket: 114 Operation already in progress
Oct 28 21:38:07 pppd[5020]: Connected to 5c:83:82:c8:2e:c5 via interface eth0
Oct 28 21:38:07 pppd[5020]: Connect: ppp1 <--> eth0
Oct 28 21:38:08 pppd[5020]: PAP authentication failed
Oct 28 21:38:08 pppd[5020]: Connection terminated.
Oct 28 21:38:08 pppd[5020]: Failed to disconnect PPPoE socket: 114 Operation already in progress
.
.
.
.
.