This is exactly where you need to start. Does your mobile carrier support mobile termination aka does it allow inbound connections? Even if the IP address looks to be public, this doesn’t necessarily mean that it will support inbound connectivity. Inbound services usually costs more, and may require a unique APN (such as ‘i2gold’ with AT&T or ‘VPN.COM’ with Rogers).
Other issues could be your firewall (from the PC or Router originating the connection attempt to the remote modem), so setting Port 80 for AceManager may work around that issue.
Remember the modems have a default Telnet interface on port 2332, so using a telnet capable client like Putty is another great test to confirm if inbound services work. PING is not a good method, as a carrier may allow inbound services, but disallow PING through the network, not to mention it can be configured in the modem. But if you can ping OTA, then you have mobile termination.
If you continue to have issues, discuss the issue with your reseller or vendor, and request updating to the latest firmware - 4.4.2, and perform a configuration reset by holding the physical reset button, then reprogram the modem settings.