My fingerprint isn't longjohnnie@yahoo.gr

I have 3 Ulefone Gemini's in my family.

2 Devices have the same problem, from one day to another the fingerprint just stopped working. It's impossible to even set new fingerprints, it doesn't respond. Factory reset didn't help, clearing cache partition neither.

I was wondering how i could open the phone (no visible screws and full alu body...) and see if it is some hardware that disconnected... So far no luck on opening the phone.