M9 Doesn't Record in Stereo Mode
When using a camera or device with a dedicated mic jack, the stereo mode will work for sure.
However, in order to have stereo recording on a phone or device that has only a 3.5mm combo jack or only a USB port, you need to get a Type-C to Type-A/C/Lightning audio cable.
Plug the Type-C end into the bottom Type-C port of the M9 receiver, then plug the other end into the USB-only or 3.5mm combo jack device. Though it's not wireless anymore in this use case.
The Audio Cuts In and Out When Recording On the Phone
It's better to have both the mics and the receiver fully charged. The low battery power does have an impact on the performance
Check if the recording range is within 50m(164ft) in the line of sight.
Static Noises/Interference Issues of Recording Audio
Ensure the receiver is connected stably to your device.
To avoid noise caused by the use of an unsuitable cable, it is recommended to use the cable that comes with the M9 for connection.
How to Improve the Audio Quality?
If your voice isn't super soft, try lowering the volume to the middle level.
Use mics with the dead cat on to cut some high-frequency power, even if it's not an outdoor environment.
Don't handhold it, but clip it on your shirt.