2000 Ford Fiesta MIL hell

Hi guys, I am really in need of some advice.

So I have a 2000 Ford Fiesta with a 1.25 (1242cc) engine. Ever since I bought the car, it has displayed the MIL every few hundred miles pulling the code P0420 (Catalyst System Efficiency Below Threshold (Bank 1))

I changed the o2 sensor, and the MIL returned within another few hundred miles, basically, it made no difference. I discovered there is a company in the UK that sells a specific “o2 cheater” or “MIL eliminator” designed for my engine, at the cost of £70+. It apparently is designed for use when you use a sports cat, or take that cat off all together (which over here is illegal) but at the end of the day, it is designed to fool the ecu into thinking the o2 sensor has a perfect reading within its tolerences. I thought it was really expensive, so i searched the internet and quite a few forums one of which talking about a ford f150 truck, and that you can make your own MIL eliminator by using a 1M resistor (1MegaOhm) and a 1uf capacitor (1 micro farad). I copied the ford one word by word, seeing as the sensor wiring was identical to mine, and it worked. But the MIL came back on and brings up the following code:
P0140 – 02 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)

I’m not sure what to do now.
So I have my modified sensor, which the ECU tells me has no activity (so its dead on 1V constantly if im right?) and then the new NTK sensor which is telling the ECU it is reading too much of a fluctuation. Where do I go from here?

Would appreciate all help and advice.