They are enabled by default on the FiOS routers, normally. Are you using Automatic Channel selection? Some routers will only use DFS if Automatic is used, due to the fallback requirements with DFS if Radar is detected.
Doesn't appear to be on enabled. It never shows the DFS channels on the WiFi channel scan page, which is where the toggle to turn them on is supposed to be according to the manual (and was also on the G1100). The only way to turn it on seems to be using the FiOS app, but there seem to be lots of compatibility issues between the app and the G3100. For example, the app will let you use spaces in device names, but that will break the DHCP server in the G3100 and it will no longer assign IPs.