No SIM card installed

I recently got an iPhone 5c from a job lot . The phone has been reset but it doesn't seem to work with any SIM card I've tried(EE , VODAFONE , O2 , 3) when trying to activate the phone I get the same prompt telling me there's "no SIM card installed, please enter a valid working sim" . I thought I'd check the phones imei number online and I found that the device wasn't black listed or iCloud locked , but find my iPhone was on, could this be why no sim works? That maybe it's network locked? I have opened it up and had a look at the sim reader and can't see any signs of damage so I'm not sure if it's a hardware problem either! Any advice would be greatly appreciated as I don't know what to try to fix