My iPhone 6's wifi strength is so weak. I have full bars if I sit directly next to the router, but as soon as I leave the room I lose all bars when I could roam the whole house before? My laptop wifi still works, but my phone can't reach that far any

My iPhone is a 6.

I have already tried to:

-Update it

-Reset all network settings

-Delete as much space as possible

-Reset my router

I used to get full wifi bars upstairs on my iPhone, but now it only connects when I sit next to the router. The signal strength of my router is fine, as my MacBook still connects with full bars upstairs, just my phone cannot locate it at all if I step more than five metres away!

If I take it back to apple will I have to pay for this to be fixed? Im spending so much money going over my data!