RE: Really weak Wi-Fi signal - XPS 13 9350
Sorry for the issues you've had with that system. The driver you've installed is Version 1.555.0.0, A02, correct?
This is correct.
Since the Dell 1820a can handle both 2.4 GHz and 5.0 GHz bandwidth have you tried connecting on the 5.0 band to see how the connection is with that?
You said it's having connection issues with all networks. Could it be that the antenna leads have worked loose during shipping? Here is a link to the XPS 13 9350 Service Manual showing how to check the antenna connection.