if i understand what a wallgarden is... the only page you get when you first open a browser that has the ISP logo and tells you to call them.. if thats what it is then the mac/cert you are using is one of theirs its just that it hasnt been registered yet IE its a legit mac/cert just that the original modem that contains said mac/cert is not in service, IE no one is paying for service on said modem so it hasnt been issued a working config...... so updating firmware wouldn't work unless you are getting a legit config when you logon(one that allows you to browse the web) but if you where getting a legit config you wouldn't get that page.... try checking the config with vulture
and see if it says what firmware its trying to update the modem with then spoof it........ under stealth on haxorware page