Diagnosis
You have two routers doing Network Address Translation (NAT), which can break online gaming, port forwarding, and some video calls. The ISP's unit is acting as a router, and your ZenWiFi is also acting as a router.
How to Fix It
You need to put the ISP unit into "bridge mode" or "passthrough mode." This turns off its router functions. You'll have to log into your ISP's device (check or label for the IP address) and find that setting. If you can't find it or it's locked, call your ISP and tell them you need bridge mode enabled for your own mesh system.
If That Didn't Work
If the ISP won't bridge their device, put your ZenWiFi into Access Point (AP) mode. In the Asus Router app, go to Settings -> Advanced Settings -> Operation Mode -> select "Access Point (AP) Mode." This lets the ISP unit handle routing and the XT8s just handle WiFi.
Heads Up
When you switch to AP mode, you lose some of the advanced router features like AiProtection, parental controls, and QoS. The trade-off is simplicity and avoiding the double NAT.