tl;dr - if you previously had connection problems using Remote Preview, it should work now.
We're now running a TURN server for Remote Preview. Since Remote Preview uses peer-to-peer WebRTC connections (much like the multiplayer engine), due to the architecture of the Internet, about ~10% of the time peers cannot directly reach each other. The solution is to use a TURN server which acts as a relay: both peers connect to the TURN server, and that relays data between the two peers.
This means if you previously had trouble connecting with Remote Preview (particularly likely over cell data connections like 3G/4G), it ought to now work, since the TURN server will act as a relay.
I did briefly test this myself: before the change I could not host a remote preview on my phone using a 4G cell data connection; once I set up the TURN server, it then worked. So you can host remote previews even over cell data now! (although Wifi is probably better to avoid running up your bandwidth bills )
However it's kind of a tricky thing to test since it depends on finding peers that can't connect and then seeing if the TURN server resolves that. So I'd be interested to hear from anyone who's had trouble connecting with Remote Preview. If it failed previously, does it work now?