iPhone 5s poor wifi speed after housing replacement

In my iPhone 5s I have replaced a back housing (to not original one, it is made by a third party) and I have a problem with wi-fi. Next to the router (about 2 meters) iPhone have 3 wifi bars on and it has the speed about 21Mb/s (download), 5Mb/s (upload) – so everything is good. The problem starts when I go far away from the router. About 8-10 meters away the phone has 2 wifi bars, but the speed is drastically slow. 0.3 Mb/s (download), 0.3 Mb/s (upload).
Speed has been tested by SpeedTest app (Ookla). I also have iPhone 4 and Motorola, but on these phones I have good speed (8-10 meters away from the router) – about 8Mb/s dl, 4.5Mb/s ul.
This means that probably something is wrong with iPhone 5s wifi antenna. I ordered and installed a new one, but the problem is not fixed, nothing has changed. What can be an issue? Maybe not original housing? Any ideas?