Port Forwarding for L2TP/IPSec VPN Behind Verizon Actiontec MI424WR-GEN2 Rev. E v20.21.0.2

I've got a NAS setup with various services running on custom ports to help minimize exposure (especially to script kiddies). I've tested everything both internally and externally to confirm they all work, and even had someone at a remote location confirm accessibility as well.  Port forward configurations performed on the Actiontec are working well. 

I installed an L2TP/IPSec VPN server, tested internally and it connected successfully.  So for all intents & purposes, this validates that the VPN server is correctly configured to accept inbound connections and functioning correctly.

I logged into the Verizon Actiontec MI424WR router, setup port forwarding for UDP ports 500, 1701 & 4500.

Note: I added the AH & ESP protocols based on what I saw on the built-in L2TP/IPSec rules

With the port forwarding in place, I tested VPN externally but it didn't connect.

I've done the following so far to no avail:

1. Double & triple checked the port forwards, deleted & recreated the rules a few times to be sure
2. There are no other pre-existing L2RP/IPSec port forward rules or otherwise conflicting port forward rules (e.g.: another rule for ports 500, 1701 or 4500)
3. There was an L2TP port triggering rule enabled, that I toggled on and off with no change
   
4. Verified the firewall on VPN server had an exclusion for L2TP, or that the firewall is off. (Firewall is off to reduce a layer of complexity, but it worked internally to begin with so I doubt that's the issue.)

Since it works internally, and there are no entries in the logs on the device indicating inbound connections, I'm convinced its an issue with the Verizon Actiontec router.  But unfortunately, I'm not sure what else to try or where else to look to troubleshoot this.  For instance, is there a log on the router that I can view in real time (e.g.: tail) that would show me whether or not the inbound connection attempt is reaching the device, and whether or not the device allowed or blocked it?

My router details:

• Verizon Actiontec
• MI424WR-GEN2
• Revision E
• Firmware 20.21.0.2

Verizon Actiontec built-in L2TP/IPSec rule templates.  They're not currently in use, but are baked into the firmware for easy configuration/selection from a drop down menu.