Asterisk sip registration with Grandstream HT503

Hai All,

I need help to setup Asterisk system with a Grandstream HT 503 ATA…

Let me explain,

I have a
1.Raspberry pi installed asterisk.
2.Grandstream HT503 ATA 1FXO and 1FXS
3.GSM fixed wireless terminal

My set up is I connected the raspberry and Grandstream through LAN port ,before that i changed the Grandstream as a Router and connected the fxo and fxs port as witha lan and a phone
This setup I need implement to use in astersik in my car

hear Am facing a problem my sip extension for FXO and FXS registerd in asterisk but its always shows UNREACHABLE

I avoided the WAN port as i dont have any local area network and internet

I connected the Ethernet port with Raspberry only.

if I connected to WAN port with a localnetwork its show reachable

But I dont have that as Am using this setup in My car.

Please help me to do setup