Set LAN to PPPoE, now cannot access router

I flashed OpenWRT to a Linksys WRT54gv2 router. The setup was successful and I have been using the router as a DNS server and firewall ahead of another router acting as the gateway to a private network. It was all working fine until...

Not understanding much about routers, OpenWRT, networks, etc., I was nonetheless curious (uh oh) about the potential of using PPPoe for the connection between the two routers, with the OPenWRT/WRT54g requiring a username and password to connect, thus requiring the other router to have this information in order to establish a connection. I set up the OpenWRT LAN network to use PPPoe as a experiment. It seems to have worked far too well. I am no longer able to access the OpenWRT router by any means I'm aware of. Further, attempts to access the failsafe mode have also failed (pushing the button, or repeatedly pushing it at various times during the boot cycle has no effect whatsoever. The router no longer even appears at it's previous IP address, nor the standard, which isn't surprising as I apparently am unable to establish a PPPoE connection, which I presume it now requires.

Perhaps I'm doing something wrong with my attempt to access failsafe (unplug, set computer to static ip, connect computer to "Internet" port, plug it back in, wait until power light flashes, press reset button...). Or, perhaps PPPoe prevents failsafe from working? I'm at a loss and I'm asking for help from anyone who may be able.

Which OpenWrt version did you flash to the Linksys WRT54gv2?

With LEDE 17.01 on the Linksys WRT54GSv1 (a similar device), there were several issues with failsafe mode: FS#790, FS#792, FS#793.

They should be fixed in the development snapshots, the 18.06 RC1 release candidate, and in the upcoming OpenWrt 18.06 release.

Now let's see what your options are to regain access to the device.

Have you used the TFTP installation method on this device yet?
Did you add a firewall rule allowing SSH login or LuCI access on the WAN interface? Any VPN setup?
Do you have a copy of your LAN interface's current configuration, and could you set up a matching PPPoE peer?
Would you be able to attach a USB->TTL serial cable? This might require soldering.