NAT issue with home DIY Router... Can PING Gateway but not WAN, Router sits in front of Gateway.

Hey all,

I've had this custom DIY home server / router going for over a decade and I finally decided to get around to upgrading my wireless NIC. The first NIC I tried didn't work so I upgraded my server from Ubuntu 16.04 to 18.04, hoping the later version would have improved support... it didn't.

So, at 16.04 with the older NIC, that's the last time it worked correctly. New NIC arrives, I'm already on 18.04, and the wireless device comes up as wlp3s0 instead of the usual wlan0. I tried renaming the device and got frustrated (was tired) so I figured it would be "easier" to just reconfigure everything... how hard could that be right?

Well I've got everything reconfigured, I'm just spelling this out to say that since it was last functioning I've gone and upgraded the OS and reconfigured about a half-dozen config files, so its entirely possible something got borked along the way... but this is an odd problem I don't believe I've seen before.

I've got DHCP working and Wireless AP functionality going, it's connecting... but the connected devices are saying "no internet". Hitting up a command line I'm finding I can ping the wireless device (192.168.2.1) and the LAN's NIC (192.168.1.1) and the WAN port on the router (DHCP)... but most-suprisingly I can also ping the gateway which sits behind the router.... the Comcast Cable modem. I currently have it setup in bridge mode though, so it's got an accessible IP of 10.0.0.1, but it's basically relaying it's WAN IP over to the router.

So I'm really struggling to figure this out... maybe I'm rusty... but how the hell can I have a NAT issue on the router which somehow doesn't prevent me from accessing a device beyond it?

Shouldn't this indicate a NAT issue at the Gateway / modem? But how could that be if it's in bridge mode?

At any rate, I could really use some suggestions... I'm all outta ideas on this one.

Thanks!