Traffic may be reaching the internal network, but it may be using a different route than you think to get there.

Use a browser to try and access the Internet.

If needed, allow traceroute and other diag ports to be opened until things are working properly. Then remove access for them again.

Look for slow hops on the traceroute, or pings to a location, as they may indicate network loops that need to be fixed.

Any locations that have an unresolved traceroute or ping must be examined and fixed.

Use network packet sniffing to ensure traffic is being routed as you expect.