Just so I understand your setup, your connections are like this:
Arris TG1672G
|
|----- Linksys
|----- pcWRT
|-----
|-----
When you connect your PC to Linksys and do Shields Up test via the Chrome browser, all ports are reported in stealth mode.
When you connect your PC to pcWRT and do Shields Up test via the Chrome browser, all ports are reported closed, except for the 7 in stealth mode.
I’m thinking that the results should be the same either way, since connections from the WAN side arrive at the Arris gateway first. Unless the Linksys is a DMZ host. Can you check that?