Truck won't start

Make sure battery has a full charge and connections all good. Do the dash lites and other electricals work? If the vehicle losses battery voltage, like when you disconnected the battery cables, it may trigger the security system. My info says the security system will disable the starter motor among other. If that is the problem, you may need a scanner that can communicate with that system, to fix. With testing, it may be possible to see if starter motor will crank the engine, but, that doesn't mean it will start. If you try that, make sure tranny is in park or neutral and parking brake is set.

The battery is fully charged and the connections are good. Yes all the lights do come on and all electricals work. Would the thert device allow the car to start up some times. Because it starts some times but then it won't. I have also tried switching the starter relay with a less needed one.

I don't know if security system could be intermittent? You would need to do testing when problem is ongoing. You could pull the starter relay, use proper jumper wire in cavities for the switch side of relay, I believe those terminals to be 30 and 87, the starter motor should crank the engine. If you try that, make sure tranny is in park or neutral and parking brake is set. If you have the key on, just where the dash lites are on, when you try this, it may start and run if problem is not the security system.