Are both cylinders valved IN?
Indicator shows RED when the in service cylinder goes empty...then it auto switches to other cylinder.
Turn lever to other cylinder and indicator will turn GREEN IF other cylinder is valved in.
Disconnected cylinders will not free flow propane...safety devices.
Cylinder has to be connected to pigtail to open the internal valve on cylinder fitting
And if connected to pigtail but disconnected at other end......the excess flow check valve inside the pigtail ACME nut will trip/close so no propane can flow.
If both cylinders are valved in service and both are full (or even one full)
Close the valve. Turn lever to cylinder you want to use
Slowing valve cylinder IN.
Indicator turn GREEN?
IF it does......light off stove top burners to get propane flow established and place a demand on Regulator.
Then check flames....steady, no decrease in size as you turn more on.
If indicator doesn't turn GREEN then pigtail or regulator could be at fault.