Asterisk and SIP terminals are all behind Nat. Solution

The network structure is as follows:
Asterisk (192.168.1.99) <--> wireless router (Public IP: 55.66.77.88) <---------------------> wireless router (Public IP: 21.31.41.51) <--> eyebeam (SIP Soft Phone: 192.168.12.43)
That is to say, both Asterisk and SIP terminals are behind Nat.

The solution is as follows:
1. Modify the SIP Extension settings in the SIP _. conf file.
Nat = Yes
Qualify = yes; it seems this item is not required
Externip = 55.66.77.88; change to match our external IP address
Localnet = 192.168.1.0/255.255.255.0; change to match our network settings

3. Modify in RTP. conf
Rtpstart = 1, 10000
Rtpend = 11000

4. Modify the settings of the wireless router where Asterisk is located
Send the SIP port 5060 and RTP Port 2017-11000 to the Intranet IP address of the asterisk.