Hello, my name is Dan.
I will do my best to help you, the only stupid question is the unasked one.
Have you updated the TCs software? That is something you did not mention and could be a part of this.
I have an APE dual band connected to a TW cable ISP with a similar configuration.
I would try first checking the software using the Airport Utility found in Utilities.
If there is one it should tell you so, if there is please download and install.
If it does not find one then unplug the TC and reset it that way, if that changes nothing then try restoring to factory settings (take note of any setting you want to reinstate).
You have bypassed the switch and seem to have partially ruled that out but it could be part of the issue.
I would check for new TC software, reset the modem (they can get stuck on stupid), reset the TC and then restore to factory in that order. Try each alone so that what works is known.
Try that and let me know.