Cox does not block port forwarding. I don't think there is a way with a stand alone modem. Is the port open/working when you connect direct to the S33? If so, do you have another router to try?
Also, why did you change the Linksys IP address? The modem is 192.168.100.1 so there should be no conflict with 192.168.1.1. Do you have more then one router or have a gateway connected anywhere in your network?
Not saying it's your router but do you have a different router you can try? Just trying to isolate the problem. Also, did you try bypassing the router? Could be a port Cox blocks locally. Isn't on their blocked port list but I have heard of some people having problems with their local headend.
Also, it might not hurt to post this on DSLReports to get more views. Post what area you are in and see if you find someone from there. Also, see if you can get a response from Luke_. He is the last Cox employee on the forum so perhaps he might be able to help.