Having the same issues. Alexa app recognizes my STBs, I can pair with the Echo Dot in the same room. Alexa responds to voice cues as if channel changing commands are successful, but nothing actually happens.
My setup: Verizon Quantum Router acts as DHCP server and Default Gateway for my home subnet. I have a TP-LINK wireless router that is broadcasting my SSID, on which the Echo Dot resides.
Is it possible that the network traffic required to get from the Echo Dot (on WiFi) is not getting from the TP-LINK router to the Set Top Box (which is connected directly from the FiOS Quantum Router via coax)?
I will try getting a Verizon tech to hard reset my STBs when I get home tonight.