Original post by: Minho ,
Considering all the things you have tried, it may well be a problem on the logic board. It could be some poorly connected antenna related wires and flexes or a damaged housing so you could open it up and verify the integrity of all the connections. The iPhone 5C is also prone to micro-fractures due to the weaker plastic housing. This damages solder joints and sometimes even the pcb. The IC U8_RF is responsible for WiFi & BT functionality on the iPhone 5C. The problem could stem from a bad IC or solder joint following an impact or it could have to do with some of the surrounding circuitry. In your case, this could be a latent problem from well before you received it (or the reason the seller wanted to unload it). Buying used phones online is always fraught with risks. Is it too late to open a dispute and ask for a refund? If you are stuck, then I would recommend you seek out a repair shop that does micro-soldering repairs. Considering the age and the re-sale value of an iPhone 5C on the secondary market in NA, it may not be worth fixing. You could always sell for parts to recoup part of your cost. Sorry, wish I had a better answer.