I bought this car about 3 month ago from my friend and right before I bought it my friend chanched the battery. It was okay for sime time and then the fob was not working properly. Sometimes I had to click it many times to lock/unlock and with the time it kinda stopped working so I was  just locking the car manually. And everything was fine until the moment when I tried to start the car and all of the lights on a dashboards went off and the car didn't start. When I opened the door the alarm went off and I couldn't turn it off because of the fob. So I tried changing the batteries in the fob, tried different reciever, went to mechanic and he said that the alarm system is aftermarket and not matching the model of the car (whatever that means). I tried to disconnect a battery and connect it back, alarm goes off and then stops but it helped to start the car and it was running with no problems for like 3 weeks until today. Same problem, disconnecting the battery didn't help. It's my first car so I don't really know anything and would appreciate any help.|
P.S. As I Have figured the car goes in anti theft mode and that's why I can't start it but if it's the problem then how can I fix it?