Nintendo Switch revised model cannot connect online or has strict NAT

The checks in more detail

In docked mode the dock's back blocks some signal, and a console behind a television in a cabinet suffers most. Restart the router and the console, and in Internet settings, choose the network and Change Settings to check DNS, and try setting DNS to automatic or to a public server such as Google's 8.8.8.8 if the test fails at the server stage. NAT type matters for online play: the connection test shows it, and a strict NAT type causes trouble joining matches.

Opening or forwarding the ports Nintendo lists, enabling UPnP on the router or putting the console in a DMZ temporarily helps, and a double NAT from a provider router in front of your own should be fixed by bridging. The Switch dock has a USB port that accepts a supported wired LAN adapter, which gives a steadier link than Wi-Fi, and Nintendo lists compatible adapters. For a home with a congested 2.4 GHz band, joining the 5 GHz network, which the Switch supports, improves speed. If a particular network never connects but others do, forget it and join again.

If the console connects to nothing, update the system software if you can, and contact Nintendo. If the console connects and then drops during play, look at the router's power-saving or band-steering features, which can move the console between bands and break a match, and turn them off for a test. A router that is also serving several streaming devices may be short of bandwidth in the evening, so test at a quiet hour. Keep a note of the error code from Test Connection, since Nintendo's support site lists each code with its cause, and the code often names the stage, such as DNS, that failed.