Not necessarily: DHCP gives out IP addresses to clients on your network, DNS resolves names to IP addresses. While both are needed for HTTPS to work, once the communication between the computers starts they are no longer involved.
My first instinct tells me this is a Comcast issue, and that they are routing your Internet traffic through a transparent proxy that fucks things up. Comcast is known to intercept at least DNS traffic, even if you set up different DNS servers.
This would be debuggable with an SSL tool, e.g. OpenSSL's s_client and comparing the output to someone else's ISP.
You can also test this yourself: Select a VPN provider (any reputable should do), set it up on a client and connect to the VPN service there. This should cut Comcast traffic filtering out of the loop, and prove that the clients are not the issue: Try to open the sites that didn't work before through the VPN and compare.
As a first step you can check how the traffic is generally routed with tracert on the command line: tracert
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