Welcome to Android Central! It would help to hear a few more details. What version of Android? Is it rooted or unrooted? How are you trying to transfer files? Does the phone even register the microSD card as present? (Check in Settings>Storage to see.) Did the wi-fi problems seem to start when you started using the microSD card, or does that seem separate? Does the wi-fi only have problems at home, or even at public wi-fi spots like local cafes or libraries?