I have a TP-Link MR3220 3G Router. I'm using this Router with a 3G USB Modem to connect to internet.
The problem with this modem is: I cannot ping the modem from outside to its DDNS address, even after disabling all security features like SPF filter etc. Since I cannot access this modem from outside through its DDNS address, I'm not able to use its port forwarding feature for my mail server.
I also noticed that its remote management feature through Web Console wasn't working properly either.