|let this be a lesson to anyone who wants to buy a linksys WRT54G router, don't get it if your ISP has a subnetmask range of 255.255.254.0 because linksys routers (or at least the WTR54G) does not support that subnetmask range.|
Unless you called Linksys support and they told you this, I'd have to say you're dead wrong. I'd bet you didnt' call them and are assuming.
I've had opportunity to work on several different Linksys models and I've never heard of one, nor worked on one, that didn't allow changes to the subnet mask. I use many WRT54GL's here at work as wireless access points for our public "guest" wireless network and they're all fully configurable.
It seems more likely there's something wrong with the unit you have. I can't see Linksys (which is owned now by Cisco) saying to themselves, "Ok guys, just for grins and giggles, we'll make it so the WRT54G can't use any subnet mask except 255.255.255.0 but we'll make every other model me manufacturer fully configurable."
Think about it, does that make sense to you?
From the sounds of it, there's either something physically wrong with that unit or the firmware is having issues. If you haven't yet, try updating to the most recent firmware. If that doesn't fix the issue, or you already have the most recent firmware version available, try a 3'd party firmware like Tomato or DD-WRT If none of this works, then something is physically wrong with the unit.