My guess is you need to fiddle with the firewall settings at the remote site(s) where the extensions show up UNKNOWN.
If the device registers on reboot and then disappears from the network you'll need to disable SIP ALG on the REMOTE routers. You may also have to disable the SPI firewall and turn off "block anonymous Internet requests". Lastly google the specific router model number to check if they are compatible with SIP.
If you have control over the routers you can also open up the ports in the firewall. TCP/UDP 5060 and UDP 10000-20000