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Don't throw away your old PC—it makes a better NAS than anything you can buy
(www.howtogeek.com)
This is a most excellent place for technology news and articles.
Buy another router that allows you to run openwrt or anything else you fancy, and use the locked-down one just as a gateway to the new one, problem solved. My setup is somewhat similar -- locked-down cable modem router that I can't customize, bought a netgear router, installed freshtomato on it