I have recently upgraded my router from a Linksys E3000 with DD-WRT to an ASUS RT-AC87R with stock firmware and since then, I can't host a Garry's Mod server. No one outside my LAN can connect. I did a port check with canyouseeme.org and it claims the connection timed out. I have allowed the server executable itself and the port it's trying to use on my firewall to no avail.
Router Config: http://i.imgur.com/XPu8Dh7.png
CanYouSeeMe.org Port Check: http://i.imgur.com/Yf3m1hS.png
Have you tried restarting the router? Sometimes if it's a stateful firewall it won't flush the states when you make new firewall rules, so it will continue to block the port until the state table is flushed even though the firewall is configured correctly.
There is no reason this should work but try rebooting the modem. I've had the modem start dropping traffic before. They're usually pretty transparent devices but mine occasionally goes nuts...
Not entirely sure. I've hosted my fair share of Garry's Mod servers and I've hit problems, none like this. Did you ever try using 27015-27016? That''s all I can say here.
You're running this on Windows? Try disabling Windows firewall completely for a bit and then try it. Maybe your router is forwarding fine but your Windows firewall is timing everything out. I've had mine do that before, I usually just add a rule to allow traffic on that port always. If you're not worried about security at all you could just leave it off, but thats not the best idea especially if you have a laptop and it isnt always on a secure network....