Make sure battery has a full charge and connections all good. If your security system has a problem, it can disable the starter motor. Any applicable trouble codes?
You see the wiring diagram for starter motor. With testing it should be possible to see if starter motor will crank the engine, regardless of security system, but, that doesn't mean it will start. If you try that, make sure transmission is in park or neutral and the parking brake is set. Make sure the vehicle can't move. I don't know what testing you have done? As you can see in diagram, the transmission range sensor is involved.
You can unplug starter relay, use jumper wire between terminals on switch side of relay, terminals 30 and 87, terminal 30 is hot all the time and fuse protected. It should crank the engine as long as starter motor is ok--wiring circuit ok and the engine isn't seized. Just my opinion.
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