Does Xfinity uses CGNAT? If not why can’t I access my machine using public IP?

I’ve a personal modem and router and when I try to access my machine using public IP it doesn’t work. I've port forwarded my local machine ip/port to my external(public ip) and it seems to work when I’m connected to Xfinity network but when I use any other network I can’t connect to machine. Does Xfinity use CGNAT? I also see dual IP v4 and v6 but I don’t think v6 is public because I can't access the v6 either. How can I access my local machine from internet?

Everything as expected as long as I am using Xfinity network.
What I have tried
1) I have checked the list of blocked port and I am not using any one of those. 
2) Checked my port forwarding settings, those are correct.
3) Changing my modem to bridge mode - didn't work
4) Tried from different machine
Note: these  Port Forwarding Instructions  are for a)xFi Gateway or b) Xfinity Gateway and I don't use either of these as I have my personal modem.

Any suggestions? Thanks