We just added the sports package so I expected TCM to be back on our channel 67. It is not. We have a non HD tv with the small black box. We had the channel previously before it was bundled with sports. I cannot find a number on the Xfinity web site to call to speak with a customer rep or a webchat link. The online bot says I am in a cue but it has been over an hour with no one coming on.
If the "small black box" is a TV Adapter (DTA), see https://www.xfinity.com/support/articles/difference-between-cable-box-and-digital-adapter. It doesn't specifically mention the SEP, but it does say "you won't have access to such features as premium channels (HBO, Showtime, Starz, etc.), Pay Per View or Xfinity On Demand programming." In most areas adapters only have access to channels up to Digital Starter or equivalent. TCM was once a part of Starter, but is now in the premium SEP package.
Unfortunately the article is out of date on some points. For example, it says "... the TV Adapter has coaxial input/output only". That was true years ago, but adapters have long been available with HDMI outputs.