I’d like to just point out that problem being described by the original poster in this thread probably has absolutely nothing to do with Wifi issues.
I say this because I experienced exactly the same thing the other night, in the middle of a ride, and I am not on Wifi, my PC is connected directly to my router via a Gigabit ethernet connection.
But even more importantly, I was riding with a friend that night, who lives 650 kms away from me in a different province, and he experienced exactly the same problem, at exactly the same time as me.
Both of us suddenly found ourselves riding on our own, in a Zwift wasteland, where all humanity had vanished as if The Rapture had just occurred.
Clearly our local networks cannot be blamed for something that affected both of us when we are physically separated by 650kms.
Our automatic assumption was that the Zwift servers temporarily glitched and I assumed that our online sessions had been dropped by the server for some reason resulting in us suddenly riding alone in some kind of offline session.
Interestingly, the total rider count didn’t drop to zero, in my case it continued to report that over 440 riders were online.
To add to this, I work in IT and I immediately checked my internet connection when this happened, and there was nothing wrong with it, everything else was working, including a speed test, so my internet connection / router couldn’t be blamed for that either.
In my case, this has only ever happened once so far, but I strongly suspect it’s a server-side problem and not client-side.