Error Subscriber absent cause 20 in incoming calls with Grandstream phone

I wish to configure another phone GrandStream Budge Tone-200 that is outside local network.
With other phones I’ve had no problems. This one, register, I can call from it, but it doesn’t receive calls with error Subscriber absent cause 20. I contact my freepbx server not through SIP default port 5060 but with another that is specified on phone settings. This port then, is redirected to 5060 on local network, by modem/router.
What can I look to?

20 normally means it hasn’t registered. If it has registered, it means you have enabled qualify but it is not responding.

and so, what I have to do or to check on phone?

You would normally look at the Asterisk logs first to see if it is trying to register, and if so what error is resulting, or if is going unreachable once it has registered.

If it is failing to register,you would look at its name, the IP address it is trying to register to, the routing to that address, the password, etc.