|When you signed up with your provider, did they provide you with a "combo unit" (ie: combination modem - router - wireless access point) or did they just give you a modem?|
FWIW, the subnet mask you got from your provider has nothing to do with them trying to limit you to one router. If you have a router connected to the modem, or a combo unit, then they (your provider) really has no interest in what you're doing on the LAN side of said router/combo unit. If you wanted 12 routers behind the one connected directly to your modem, you could do it and I'm quite sure they couldn't care less.
I suspect the problem is your configuration of the routers you're trying to add. For more information on how to correctly add one, or more, routers to your existing network simply click on my name above in this response and read my “how-to” guide titled, “Add a second router to your LAN”
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.