said by Thinkdiff:You won't be able to telnet into the modem - that access is disabled. Even if you could, there's nothing in there that would help for this issue.
Can you try changing the WAN-side port in your port forwarding rule to something more obscure? Maybe 4489? Keep the internal port set to 3389. Then try to connect to yournewIP:4489
if CV is actually adding an MIB filter for port 3389 on the customer side modem, then id be pretty PO'ed about it.
there are already a slew of ports filtered on the cable modem side, specifically concerning port numbers that were used for remote access trojans many years ago that have no bearing or can even be operational at this point on current operating systems such as windows 7 & 8.
gee, lets block 6588 modem side because a remote access trojan used it back in 1998 on windows 98 2nd edition.