SIP 408 is generated when the request(call, register) cannot reach the VoIP server for some reason or the response cannot reach you. If the issue is NAT related, altering STUN and rport options could help. Both are located in your account configuration. Try altering(disable, enable, different combinations) and try to register again.
If these do not help, you should contact your ISP or network administrator to check what changes have been made to the network.