So I've recently tried to purchase the Ooma VoIP adapter to add as home phone to my network. 

I initially had problems out of the gate. I was able to finally get the Telo device to register correctly by directly connecting it to my ISPs ONT. However if I connect it on my network it fails to establish a connection. 

I was originally in standalone mode and went to a software controller and adopted as I was seeing that there was more granular control from the Omada panel. However everything I've tried has failed . 

I've tried the following: 

The ISP and ONT pass the traffic perfectly when directly connected.

The MTU is a clean 1500.

Every anomaly, flood, and SPI firewall defense is disabled.

Hardware offloading is off.

The ports are hard-forwarded, and the native LAN DMZ still fails.

VPN Passthrough (IPsec/PPTP/L2TP) is enabled.

Contacting Ooma support basically told me that they're seeing intermittent connection but they're failing to see the VPN via OpenVPN be actually established properly. My latest research is showing that it could be the NAT rules and the suggestion from AI was to enable full-cone NAT which there doesn't appear to be an option to do so. And there is no way to directly edit the firewall rules.

I tried using the device as the initial router but even at that connecting it to the WAN port of my ER 605 failed to actually allow it to get to the internet. Although this would have been solely a test as my reason for having the 605 is to have multi-WAN functionality.