I have a AT&T Gigabit modem/router (ARRIS BGW210-700) configured in IP passthrough mode with a Google Wifi router. I have to go with IP Passthrough as it is not possible to put these devices into Bridge mode. I have ports forwarded on the Google Wifi router to a server on my network which is hosting a few web applications.
I am seeing that external access to these applications are intermittent. For example, in a web browser, I can try to access one of my apps using my external IP and the external forwarded port. The site will load, but if click refresh repeatedly, eventually it will fail to connect. If I keep refreshing, eventually it will load. I have not been able to get a consistent, external connection to my applications through the forwarded ports.
I have confirmed that during the times that I am not able to externally access my applications, I am not having any issues using them through the local IP.