How far is your range extender from the Main router ?
As for pc connecting or phone connecting to the extender instead of the main router you need to make those device "forget that network" but if you have both configured in the pc and phone, they will decide best quality line signal and connect to that signal first.
I would suggest setting up the SSID of your tp-link range extender TL-WA854RE to be identical to your router (same name) that way instead between choosing between 2 networks, the system will choose between closest /strongest signal and switch between the two as you move about the house.
But if your tp-link range extender TL-WA854RE is overlapping on your router signal to prevent device to connect to the main router;
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