I am not able to authenticate via OAuth authentication to ServiceM8 api. Getting invalid client credentials

Hi ServiceM8 api team. I am a freelancer. My client want to build his own app to jobs and some other stuff. Now to get the jobs, staff and customers data into that app we need to use servicem8 api. Because of 2fa we have to use OAuth authentication. Our app will access the api on a PHP(Laravel) backend server. So we are not able to use authenticate via Oauth. please help. I have created add-on and followed all the step described in api reference. Please help as soon as possible.