Here are some tips for anyone looking at remote control extenders. This is based on what I've read on AT&T U-verse forums over the last couple of years. U-verse uses the same hardware and software and has over 2 million subs, so it's a good resource for more unusual situations.
The Mediaroom set-top-boxes use a carrier frequency of around 60 KHz while many IR extenders only repeat frequencies in the 40 KHz range. One product that users have reported success with is the
Hot Link Pro.
I haven't tried a remote extender personally.
There is also a thread in the Bell Fibe TV forum where someone tried a standard remote extender unsuccessfully before getting everything working with the Hot Link Pro.
My guess is that the old remote used 40 KHz, the new remote uses 60 KHz and your extender doesn't support 60 KHz.