Using a range extender to extend one's existing WiFi signal may not be as good as it was advertised (there are factors affecting the result). I think we need a relatively powerful primary router and a good enough range extender for things to work.
I'm using R7000 with TP-LINK RE350 range extender. After testing the range extender's RE and AP modes, I've decided to use a long Cat 6 cable and setting the RE350 to the later one. I'm also using different SSIDs to distinguish between R7000 and RE350. I wanted to know which device I'm connecting to.
Try to put the router in a higher position if you can't relocate to a better location. I've saw some people just put on the table height or behind the TV. You may use a longer lan cable to free the router and test which spot can give you the strongest signal.