I am a business FIOS subscriber with 5 static IPs. I have the following network setup:

Verizon provided ONT > Dlink switch > Dell server running Ubuntu 12.04 with iptables enabled and a static IP address.

The makes/models of hardware are:

FIOS ONT
Alcatel-Lucent I-211M-H ONT

D-Link
D-Link Web Smart Switch DES-1228P

Server
Dell Optiplex 755 (Ubuntu 12.04 Server)

I have iptables running on the server with http, https and ssh ports open. I can connect to a website on the server from an external computer, but after a certain amount of time (mins to hours), I can no longer connect. All I have to do to re-enable connectivity is connect to the server via SSH from a computer INSIDE the network. I don't have to actually login, I just have to establish a connection. I can then access the website externally again.

I did some googling and it seems some of verizon's equipment had an ARP bug where the ARP entries would expire after a certain time period, but those issues all seem to be from back in 2009 - 2010. I know the switch has an 'auto learning Mac address' feature, but I'm not sure if that could be the problem or not. Does anyone have any ideas or advice on how I can troubleshoot this?

Thanks!