I need to disable IPV6 for IPV4

It's more likely the product or reseller is suffering from bad engineering documentation or translation, and they were really trying to communicate a WiFi radio standard compatibility, but didn't understand how to do that or what the issue was.

There is no setting to disable IPV6 on Xfinity gateways.  

If you can find the standard the WiFi radio in the product uses (should be on the box somewhere, it will say something like 802.11b), and the issue is the radio in the product isn't compatible with WiFi 6, you can change WiFi standards the gateway is using the phone app. 

Xfinity gear is poorly documented too.  The idea is you enable a 2.4G and 5G broadcast sacrificing WiFi 6 throughput for radio chip set compatibility.  Others  say the local admin interface on the gateway no longer works, so you have to use the phone app to do it now.  This is it for documentation and it's really dated, unless someone has a better link or another post put together better documentation.  Xfinity refuses to do it:  https://www.xfinity.com/support/articles/change-wifi-mode-admin-tool-xfinity-xfi  

You can also just buy a 802.11 b/g/n N300 access point if you have older gear/IOT gear and it will work with most things out there.  They're cheap.  You might be able to get a 3rd party gateway, or an Ethernet router with multiple radios that can do all WiFi standards at the same time, but it will be  expensive.  There are better ways to do it.

@user_c17f7e Thank you for reaching out on the Xfinity Forums. Our community member has provided some advice to you. Please let us know if you still need assistance. Please also feel free to check out How To Change Your Home Network's WiFi Channel Selection or Mode.