Not sure if there’s anything that can be done to be honest.
I’ve checked google, some other people posted the exact same issue about port 6667 being blocked in Malaysia for “safety” reasons.
Only your ISP can unblock it, port forwarding won’t do any good since they force the blockade.
It seems to be an issue for years in your country.
Unless ToxicShock has ideas to get around this blockade?
The only way I think is using an VPN server near your location to reduce latency.
Since I still using ADSL Broadband (DL:8mbps UP:512kbps) latency not that good, using VPN make things worst.
Another workaround, working: 3v3, chat, invite... (co-op not yet), using Proxifier software.
There are some Public SOCKS Proxy that work with port 6667
Public proxy server. You can find public proxy via Google, better get a SOCKS proxy since it able to tunnel UDP just in case, always use "Check" button to test connection.
Proxification Rules. Move port 6667 to SOCKS proxy, make sure "Default" rule name use "Direct" action. You wont let random pick proxy able to see your activity.
Test if proxy server allow 6667. Then, Connection is established.
I hope ToxicShock make an update that use alternate port
NOTE: You can use any free proxy program like WideCap. Proxifier is paid version. EDIT: Add some information.Go to the full post