Hello! I for sure have a double NAT problem - after writing this post I went to connect my xbox to the new mesh WiFi and it told me this was the issue, but I had no idea how to fix it until all of the great advice on here. Your instructions were spot on and now everything is working as expected! :)
Do you have any thoughts on if I should stop the ISP router from broadcasting its 2.4 and 5GHz networks now? They seem redundant, but I saw that turning the router into modem mode will prevent me from using all it’s Ethernet ports :/