When it first became apparent that many people could not connect to the WiFi, a number of suggestions were flying around as to the cause. It became folklore that the cause was mixed 2.4 / 5 GHz networks but without any real effort to confirm this. Partially, this got traction because people discovered that if they created a guest network, it would then work. But I believe this worked for a different reason...
I didn't have my car at this point but I suggested in a couple of places disabling WPS (I believe I was the first person to suggest this). The reason I suggested this was because earlier in the year I had exactly the same problem with some M2M WiFi modules we were using on a project. Where WPS was enabled they simply did not see the networks at all. What don't guest networks have enabled... WPS. That's why that other 'fix' appeared to work in my opinion. I still don't know why WPS would cause this - there's not much logic to it.
This was demonstrated to work widely. Almost everyone who had a problem connecting found they could then connect with WPS disabled.
I then got my car, and the thing that was immediately apparent was that I had no problems connecting whatsoever. I didn't change anything on my network - it just worked. This was at a time when virtually no-one could get the WiFi working without fiddling, so I set about trying to figure out what the root cause was.
I run Ubiquiti UniFi access points, not normal domestic stuff, and there is no WPS. So that certainly helps confirm that theory.
I have three networks in my house - my generic one which the vast majority of devices are connected to. This is a dual band 2.4 / 5 GHz network. The car connected to this without any problem. I have a 2.4GHz only network which I use to connect some flaky IoT stuff to. The car connected to this without any problem. Finally, I have a dedicated network for a couple of game consoles which isn't NATted - a fully routable public IP is used. The car connected and worked fine on this one was well.
So the next test was to rummage through my box of old stuff to do some more testing.
1) Technicolor TG582n router - this is a single band 2.4GHz only router. Car did not see the network at all with WPS enabled. Car did see (and successfully connect) as soon as WPS was disabled.
2) Netgear WNDR3300 running dd-wrt - this is a dual band router, there is no WPS functionality in dd-wrt firmware. Car saw it and connected fine regardless as to whether or not both SSIDs had the same name.
3) Netgear WNDR3300 after full factory reset and running the original Netgear firmware. This re-enabled WPS. Car did not see this with or without split 2.4 / 5GHz networks. Disabled WPS and it then worked normally.
Now I appreciate this isn't exhaustive or scientific, but I'm personally satisfied to state with an extremely high probability, that it's WPS that causes the issue, and WPS alone.