This is a guess, worth precisely what you paid for it:
Many modern Wireless Access Points (WAPs) have a security feature whereby the WAP will prevent all clients connecting to that WAP from seeing or talking each other, to prevent hostile clients from snooping traffic or probing/attacking machines on the same subnet. This makes sense on a public WAP where you don't want your Linux laptop attacked by the bell-end over in the corner who brought in their compromised Windows machine.
However, on a private, WEP2-secured network whose clients are fully patched, it makes less sense. I don't know if Sonic's Pace/Arris modems have this security feature (I visited those menus just long enough to turn off all the radios), but you may care to poke around in the config menus to see if there's a feature like that in there, turn it off, and see if that helps.