I use elastix 2.4 and i get access in side lan is work perfectly but when i try to access from outside lan it perform very slow that make me sleepy when i wait for it.
Maybe try to launch some pings and check if there's any jitter, latency, ip packets dropped....Etc.
Check your default router, nslookup, and using traceroute for testing the WAN access.
You may check the IP trafic if you are able to do it.
If the Elastix server works fine on the LAN, it should works on the WAN also.
I doubt that the problem come from Elastix server to be honest.
You can try to check some wires if the connecting is under 100 Full everywhere and not a device on Half duplex connected on Full duplex.
Check the bandwidth side WAN too.