When you use a vpn, you encrypt the traffic between your vpn client and its connected destination server. You create a tunnel in which the cable company cannot see.
In order for spectrum to deliver video programming via the Internet to you at home, the stream MUST pass through the modem attached to your video account unencrypted. (Outside the tunnel your VPN creates). This "passing through your cable modem" is how they know you are at home. You can only take advantage of this benefit while at home. Using the VPN puts a mask on your internet traffic, and spectrum no longer knows you are at home.
If you want to use Spectrum IPTV at your home via your pc, roku, or iOS device, that traffic must pass through the modem unencrypted.
Turn off the vpn.