Have you factory reset the XR router and power cycled the ISP Modem OFF for 1 minute then back ON? Then walk thru the setup wizard with the XR. Input 1000Mbps if you manually set the speed or let the wizard check the speed for you, what does the setup wizard report finding for it's test run here?
Be sure to use WAN port #1 on the ISP modem for the connection to the XR router.
Be sure that ALL devices are disconnected from the XR router accept for 1 wired PC.
First run Ooklas installable speed test app on your wired PC soon after the wizards completes. Or disable QoS, reboot the XR router then speed test with the wired PC.
Some wifi devices will be limited to 400-500Mpbs over a wireless connection for speed testing.
Check your connection rates on your mobile devices with wifisweetspots.