He prolly thought you were thinking you had a 100mps connection speed. but your network is at 100mps (LAN).
Im not familiar with your modem. It shouldve come with a manual, and if it has a router and/or firewall built in, it owuld be similar to your linksys.
My guess is you do not have a firewall or router in the modem... (or else youd have spent some time getting both the modem and linksys to work together to get you internet in the first place)
Check your logs in your firewall to see if its blocking communication from your friends ip address. Otherwise, one thing you can try, is to disconnect your router. make note which cables plug into where. Then plug only your server network cable into the modem. reset/reboot and all that. This puts just your server on the internet during this test, and the firewall is no longer involved in the process. Then have a friend try to connect to your server. Make sure all firewalls, AV, etc is off.
Once you establish whether your friend connects or not, disconnect and reconnect your network cables as before. You dont want to leave your server plugged into the internet unprotected for too long.
Now if the server connection worked during this test, then its something in your firewall not allowing traffic through.
If it didnt work, then its your ISP or something still blocking within your pc (AV, Firewall, etc)