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The bars you’re seeing on the left is the signal quality received by the flint from those device. The value in dBm represents the same thing (-40 is good, -95 is bad)
If the flint shows a low signal for your TV, it’s probably that its WiFi chip has low TX power.
As for the phone signal changing when you’re using it, know that for 5Ghz network even having your hands next to the device antenna can lower the signal.
Also some WiFi chips in those device adjust their tx power according to the signal strength from the router to save energy.