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Use a proxy?....on....a...cell phone? I'm not sure how easy that will be, since most cell providers don't accomodate for a browser that allows you to proxy a cellphone.
All I can say is go to 4chan's site and go to a board it says you're banned on, get the IP from that page, and mail banned@4chan.org pleading your case. That would most likely open the floodgates for whoever got your provider banned, so if it doesn't stay open for long that's probably why.