[Modem] Admin page blocked on my SB8200?

I recently bought an SB8200. For first few days I could access administrative page, but starting yesterday I noticed I can no longer. Symptoms when I am cabled directly into modem: I can ping 192.168.100.1 Port 80 on modem - seems filtered. I see in tcpdump SYN packet being sent repeatedly, I never receive a SYNACK to proceed with 3-way handshake. Port 443 on modem - connection refused. I sent SYN, receive a RST packet. Port 8080 on modem - the spectrum analyzer. This works fine, just not the admin page on port 80. Any ideas? Cox Ultimate service 300/30