Under 8.10 you are still probably needing to use the FreePBX utility. It is accessible via Advanced>Phone Lines on the admin page, I believe. It has been a little while and 10.04 does not use FreePBX any longer.
There was quite a bit of information on the wiki about using FreePBX and how to get into it from web-admin. There should be links there still to get you to the FreePBX webpage for specifics on settings and configuration.
Taken from Whirpool:
VoIP
Go to wiki
FXS (ATA) ports 2
Go to wiki
FXO ports 1
Go to wiki
PSTN fallback Yes
Go to wiki
VoIP Dial Plans Yes
Go to wiki
PSTN Dial Plans No
Go to wiki
Supports Telephony features (caller ID, call forward, etc) Yes
Caller ID, Call On-Hold, Call Forward, Call Waiting and Transfer
Go to wiki
VoIP provider config presets No
Go to wiki
Number of VOIP providers supported 1
Each FXS port can have a separate VoIP account but must be from the same provider
Go to wiki
T.38 (Fax over IP) Yes
So your external router IS your ata... Do you have another external router you could use which would let you move the nb9wmaxx onto the internal network and turn its' DHCP off? Barring that I have been successful using resources on the external network but that was for fairly generic devices that once I told lmce where to find it (ie the external LAN IP) it worked but unless there is a template for a router+ata I am not sure asterisk will be able to configure it. Rumor has it that some providers will try to provide some assistance for SIP users or offer a forum where users can discuss their settings I would check there for sure if you haven't already.