RT31P2 router can't register to Asterisk server on local LAN

Hello everyone… I have a Linksys RT31P2 router(with 2 phone port), I connected Asterisk server and my PC to the LAN ports.
I can register to the Asterisk server with the softphone on my PC, but when I setup the router’s phone account with the same SIP account, the router cannot register to the Asterisk server…
What should I do to solve this problem… thanks!
------------Voice Status Infomation of the Router---------------------------
Registration Status: 8000 8000 Offline
Call1 Status: Idle
Call2 Status: Idle
-------------Voice Line 1 Setting of the Router------------------------
User ID:8000
Authentication User Name: 8000
Authentication Password: *****
Registration / Proxy Server:
NAT Traversal: NONE
----------------------IP Address-----------------------------------------
Router IP :
Asterisk Server :
PC :

I have that model of router.
Linksys have told me that it can only register the two FXS ports out over the WAN interface to an external SIP provider / ITSP.

You can’t register over the LAN ports.

I ended up just using it as a router with simple QoS - I don’t use the FXS ports on it at all now.

You could try getting an unlocked Linksys PAP2T to register a couple of FXS ports with your local Asterisk server.