Restart your Xbox console
Restart your modem and router
Check your internet connection on Xbox
Set your network to Wired if possible
Enable UPnP on your router
Make sure NAT is not double NAT
Assign a static IP to your Xbox
Set up port forwarding for Xbox Live
Open these ports on your router:
TCP 53
TCP 80
TCP 3074
UDP 53
UDP 88
UDP 500
UDP 3074
UDP 3544
UDP 4500
Remove any conflicting port forwarding rules
Disable strict firewall settings on your router
Put the Xbox in the router DMZ if needed
Avoid using multiple routers or extenders causing double NAT
Contact your ISP if NAT is still strict or moderate