it's in the remote section of the appleTV settings, your ISP should have nothing to do with it.
the most reliable way is if your TV supports HDMI-CEC, there are typically settings in your TV to turn this off so you might want to verify that hasn't happened by accident or was changed in a software upgrade on the TV, (TV manufacturers use different names, typically have link or sync in the the name though, if you google HDMI-CEC and your TV make, you should find out how to turn it on)
if your TV doesn't support CEC, then the aTV should still be able to read the make and model from the TV over the HDMI cable, and send the appropriate commands to the remote, it will then use the IR blaster on the front of the remote to control the TV. You must have the front of the remote pointed towards your TV or this will not work.
If it's not able to read the model number, then I'm not sure if you can use the siri remote to power on the TV, but you can program it so the volume commands work, it's in the remote section of settings, then home theatre control and volume control. It will use the IR receiver on the aTV to learn them, and then the IR transmitter on the remote to control the TV.
Get help using a home theater or universal remote to control your Apple TV.
support.apple.com