The pow development web server works by installing a firewall rule that redirects connections to 127.0.0.1:80 to the actual port pow runs on. 127.0.0.1 is the "loopback" IP address that's only accessible from the machine pow runs on. If you want pow to be accessible from other machines, you must edit that rule (or add another rule) in /Library/LaunchDaemons/cx.pow.firewall.plist so that it redirects from port 80 on your host's LAN IP address (10.0.1.10, I believe you said) to the port that pow runs on.
Then, make sure that another LAN machine can access your site by going to http://10.0.1.10/ .
Next, make sure a machine can access your site by going to the WAN IPv4 address of your AirPort Time Capsule. Note: This is NOT your Arris cable modem's public IP address. This is probably a 192.168.x.x address.
Finally, once all of the above are working, try going to your Arris cable modem's public IP address.
Edited to add:
If you use a LAN machine to go to the Arris cable modem's public IP address and it still doesn't work, it may be that your Arris cable modem doesn't do "Hairpin NAT" correctly; that's where a NAT gateway has to take a seemingly outgoing (LAN to WAN) packet, and realize that it actually is for its own public IP address, and that incoming NAT (WAN to LAN) processing needs to happen on the same packet, to get it to the port-mapped server or DMZ.
To see if a lack of hairpin NAT support is your problem, use an external machine to connect to your Arris cable modem's public IP address and see if you can get to your website. There are "web server testing" web apps out there, or you could just turn off Wi-Fi on your smartphone or 3G/4G tablet to force it to use 3G/4G data service, making it an external device.