You should use the original wire (or a wire from a receiver cut to EXACTLY the same length as the original before it was cut) and probably a high silver content solder just to be safe (I'm not sure how much good quality solder would help to be honest, but it's what I would do to minimize risk of the solder being the problem). Did you cut the original wire when you soldered it back on the first time? If so, this is why you have range problems. FHSS aerials have to be specific lengths, there are several lengths that work, but lengths in between cause signal degradation, it's got something to do with signal wavelength, I'm not really sure the of science behind it. If you do end up needing to replace the antenna, you will need the right type of wire, and I have no idea personally where you could find it.
www.modelsport.co.uk and
www.apexmodels.com (both UK) both sell the exact same unit under the Ansmann brand, you'll find it in their online catalogues. It might be worth giving them a ring and asking if they can help at all. They may even have a dead unit they can send you for spares.