Are you using chan_sip or chan_pjsip? This often happens when you are trying to register on the wrong port or if both drivers are enabled the port for the other driver.
Settings > Asterisk SIP Settings and do you see tabs at the top for both drivers?
So is the extension you’re trying to register a pjsip or chan_sip extension? Which ports are the drivers listening on and which port is the phone trying to register to?
If your extension is actually pjsip and pjsip is set to listen on 5060 and your phone is registering on 5060 then make sure you have applied your config and have the right extension secret set
So, after following absolutely every step except for actually paying for the service and endpoint manager. I still have the failed to authenticate and connection issues. Should i have to buy a license or anything to solve this issue? I have verified all parameters with my grandstream handsets.