@Zook115
In most cases, you do not have to. That also helps with troubleshooting when your ISP modem acts as router/gateway. If you can have Xfinity in its default router/gateway mode and no device or service complains of "double NAT" (Google it), you are all set.
The next step after that, if you don't need Deco mesh "router" features and only need X20s for WiFi coverage, would be to switch Deco mesh to Access Point mode.
See: What’s the difference between Access Point mode and Router mode on the Deco?
Also: How to set up Deco to work in Access Point mode
Note that this can only work with Xfinity router/gateway, not when Xfinity is in bridge mode. Deco Access Point mode will also eliminate possibility of "double NAT" problems.