So I recently bought a C420S2 (comes with 2 C420 cameras and a H200 hub). 
 

Local recordings to my microSD card haven't been playable on the app. Some work, randomly, but most do not play. Instead, I just get the error 'Network is unstable. Please try again later'. I had similar issues on my D320 doorbell too. The microSD wouldn't even work on my computer. 

After lots of testing, I've discovered the 2 root causes and workarounds. 
 

1) H200 format/initialise does not work

The format is incorrect, and is then unreadable on other devices. To workaround, format the microSD on another Tapo camera or as FAT32 (MS-DOS). Formatting on a macOS worked fine. Inserting the newly formatted card will then initialise it correctly on the H200. 
 

2) Local recording will not play if camera has the 'Record Sound' option deactivated. 
You'll get the 'Network is unstable. Please try again later.' error. The files will save, but be corrupt (some media players won't be able to play them, but VLC will if needed). To workaround, 'record sound' has to be on.