Proxy Server

Hi guys

I need to setup server/router in my firm. We got from our ISP dedicated server in their data center. It has a static IP and it servers as replacement for out DSL connection. I configured our internal server to be border gateway and to connects to data center. "Remote" admin installed squid and IT staff can access to the websites but other can't. I want to remove that proxy server. I think I really don't need it.

Here is the picture how above situation looks



Server router has two NICs, one is connected to ISP router which we can't access, and one nic is connected to our private LAN. I use CentOS Linux 6.5 x64 to act as router. I added in iptables firewall to forward all traffic from eth0 to eth1 (from internal to public) and allowed traffic to leave server. All output traffic is nated.

Problem is on the other site. I'm not a network engineer but only with some knowledge in networking and formal IT education at the firm. My problem is I can't ping anything on the Internet while our VPS can. On that same server only port 80 and 22 are opened. People who setup proxy can connect to outside world but can't ping, can't use other networking software.

I have some questions:

How I can see my private LAN from ISP network, I will need to setup VPN in the short time ?
Is possible to avoid proxy and just with firewall NAT get data from the net back to the hosts in the private LAN ?

I am afraid to mess with ISP server because they administrate that server, we are and I really won't to understand how configure a network.

Thanks for reading and suggestions.