There are a number of free Wi-Fi networks in Bj, sh, and hk that will render your 4g connection unusable if you permit them to auto connect.

I will use ishanghai as an example because it is the most annoying for me, but I also encounter problems with chinanet (my home internet provider, which also happens to serve many hotspots, including Starbucks).

Ishanghai is pretty easy to sign up for (only need to do phone verification once), but every single time you connect, it's necessary to accept their terms to really get online. Because its coverage is good (but, not great), it will auto connect frequently as you travel around Shanghai (thus leaving you without data service).

I occasionally use ishanghai because one of my clients is fond of a mall basement coffee shop that is served by it and has a poor unicom signal.

However, I now make sure to delete it after each use.