Oh, dear, where do we start?
Firstly, if your existing aerial was working OK and the connections aren't badly corroded, there's no point in changing it. Simply replace the cable.
Secondly, without knowing your postcode, we don't know which transmitter you are using so it's impossible to recommend a suitable aerial.
Thirdly, the eBay aerial has only 7 elements in the photo so, despite its claims of "12-14dB Gain" I'd bet that its actual gain is no more than 5dBd. In fact it looks as if it could have a built-in amplifier, which is not going to help a weak signal. The purpose of an amplifier is to allow you to use a very long cable or to split the signal to several rooms - assuming (in this case) that your signal is strong enough.
The eBay aerial is a wideband "Yagi" type, for which much of the gain is concentrated in the useless "4G" band (60 - 68 ). All in all, not an ideal choice - especially if you don't need a wideband aerial.
The cable insulation resistance should be infinity.