Just to add to this... I used a wifi meter on my phone to check signal strength and the possibility of interference. Despite the fact that everything else in the house was connecting and communicating just fine, and that when close to the wifi router the signal strength was huge, I noticed that a neighbour was using channel 6, while we were on 7. There were also a couple of weak signals on channel 1. So, I moved us to channel 11, That didn't solve the problem.
I'm loath to suggest that there's something about our SSID - it's simply the one that came with the device... but I'm struggling to think of anything else... and I don't really want to change it, because everything else will lose connectivity!
Given everything else is working fine in the house, maybe it's the Ninteno Switch that is the problem rather than the modem?
That would be a reasonable thought except another Optus wifi router is visible (at the neighbour's house) and I was able to connect to my tethered phone. The ONLY SSID that's invisible is our own one!
(we even tried to manually enter the credentials, but that didn't work either.)
The fact you can see another Optus router but not your own makes me think even more it's the switch, not the modem. But either way, if you haven't changed the SSID from the default and haven't touched any other settings in the modem either, maybe try doing a reset of the modem? It forces it back to factory settings but if you haven't changed any, yours are already factory settings so no big deal. Normally there's a pin hole in the back to reset if you stick something in there for a minute or so
Yes... the only thing we've changed is the channel. Originally it was on 1 but a few months ago (on the advice of a telephone support rep) we moved it to 7 due to interference... but the neighbour (bless him!) followed us... which is why I moved to 11 when this issue arose.
I've also raised this with Nintendo - I figured I'd run both sides at the same time... will see what they have to say for themselves...