Simply head to the official Nintendo page and see what it says under ‘Online Service Status’. Hopefully it will say “All servers are operating normally”.
You can also check maintenance information as well as maintenance scheduled for the future.
If not then the problem will be elsewhere, perhaps with your local Wi-Fi connection or your internet service provider like BT.
With the coronavirus outbreak and a huge game like Animal Crossing New Horizons being released, it’s possible that Nintendo Online might see a large amount of demand that the servers can’t cope with.
Read about how gaming is the answer to your self-isolation anxiety.