poor and weak Wifi signal

the wifi antenna connector is something I wish Apple would change, its sometimes hard to reconnect after whatever fix has been done to your phone that requires its disconnecting, I would guess it's not properly connected, I would disassemble your phone again and check to see if it's not properly connected and correct it if it's not, if you cant seem to get it to (snap) back on then get a magnifying glass and inspect the port on the circuit board where the antenna plugs in because these ports tend to get damaged when to much force is used trying to plug them back in, make sure it's perfectly round, also check the port on the ribbon cable too, also dont rule out that the wifi antenna may need to be replaced.