Have:
Garmin dual HR sensor
Varia rtl515
Garmin cadence
Bluetooth android samsung s10
Shimano DI2
Working before starting the ride. Than after a couple of minutes into the ride all these connections fail at once.
In the beginning i started tinkering with restarting and reconnecting, but i found out that if i did nothing and continued the ride alle connections come back at once after about 3 minutes.
Whats the deal here?
Same issue here!
I have the feeling that power off the device before the ride helps to avoid it (most of the time).
If it happens during riding i do following: l stop the ride and press the power button once (standby). Then i wait ~30sec. and press the power button again and hope that the sensors reconnect. Works most of the time.
It is super annoying during trainings! I always record with my watch as backup because the 840 is not trustworthy 
Sometimes it loses the sensors, sometimes it reboots during riding, sometimes the altitude goes crazy (e.g., 250m off).
