Weak wifi signal. Still not fixed after wifi antenna replacement.

This has been a problem for at least 3 months. I try to connect to a network and it says "Unable to join network". Sometimes it works but it is slow and often cuts out when I try to download something. I know its not a problem with the router or the network because I have tried this on multiple networks that work on other devices.

Have tried resetting phone to factory settings. Resetting network settings. I even replaced the wifi/bluetooth antenna and this did not solve the problem. Still give me the "unable to join network". Any help would be appreciated. Thanks.