unable to access some websites when connected to my home network

Try disabling the Xfi Advanced Security feature as a work-around for now and see. It's buggy ! And there is currently no way for an end-user to be able to whitelist anything;

https://www.xfinity.com/support/articles/using-xfinity-xfi-advanced-security

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@alistairb wrote: ... https://109.203.125.112:8443/ ... the site cannot be reached

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Note that when I try to follow that link in Firefox I get:

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Warning: Potential Security Risk Ahead

Firefox detected a potential security threat and did not continue to 109.203.125.112. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details.

Websites prove their identity via certificates. Firefox does not trust this site because it uses a certificate that is not valid for 109.203.125.112:8443. The certificate is only valid for the following names: *.specialservers.com, specialservers.com

Error code: SSL_ERROR_BAD_CERT_DOMAIN

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apologies for the delay in responding, yes, the certificate is bad...because my host has changed names and I prefer to use the IP address directly.

when I use my cell provider I get that message and since it's the same site I have used for about 15 years, I know I'm ok with it. 

If I'm connected by my home network, I don't even get that far.