yes dyndns is good certain routers have it built in all you have to do is insert your account, another solution would be to have a another computer with no-ip you can sign up and at no-ip.com and get the app for linux, windows & mac
DID YOU OPEN UP PORTS 5060 AND THE RTP PORTS? UDP/5060 -> Forward to YOUR LOCAL IP UDP/10001-20000 -> Forward to YOUR LOCAL IP DID U TRY PINGING YOUR DYNAMIC DNS? TO SEE IF IT RESOLVES TO YOUR PUBLIC DNS? IF YOU DONT NOT HAVE A sip_nat.conf create one if you need any live help you can add me email@example.com, or soulofmischief87 on aim
go to the tools tab in pbx then the file editor first check if the file exist if not create it, this is the easiest way, so you check in the file list to see if you already have if not click on the new file button then you add
this is assuming your local net is set to this network and subnet, put your dyn dns host name in externhost then save the file and in the tools section use the cli and type "module reload"
make sure you have the ports forwarded to our servers local ip and point the device in the outside network to the dynamic dns after you see it register you can use if as a remote extension