Router to Router - no wifi possible?

I bought the mt300n-v2 and it works great as a wifi to wifi router. Even works as wifi to router. However, I can’t get Internet to work using at was wlan to lan.

Here’s what I want to do:

Use the Xfinity WLAN to connect to the mt300 (no wireless, just ethernet). Xfinity will assign the IP to the small router via DHCP. This works great.

Using the LAN out on the 300, I connect to my internal router and the device assigns an IP to the big router. I can “see” the 300 and hit the management app from my computers; but none can get on the Internet.

I set up the big router as the DMZ in the 300; but no go. When I see the Cable connection on the 300 web app, it says connect via DHCP and then shows that it failed with no internet connection (my problem); but, it does have a DHCP assigned IP from the Xfinity box.

This should be simple - just use the device as a passthrough for the Internet connection. Why? I wanted to use the VPN on the 300 for times when I’m on the road.

Suggestions please?

Your logic seems a little confused. Can I have a simple topology?
What is the connection relationship between the big router and 300N-V2?
What is the connection relationship between the internal router and 300N-V2?