HGC ask PCCW for technical support, and so they change the connection between my flat (block 2) to server room (block 4). As HGC providing services with VDSL and rent the LAST telecoms connection from PCCW, so HGC can't solve the problem alone. Anyway, the disconnect problem is now improved.
I guess the problem should due to the status of my flat: the building itself is already 10+ years old, where I am the FIRST one who buy and use this flat. So the telecoms connections didn't been use for around 10 years, it should then decayed.
Well... Here is the syslog from Linksys W310N + DD-WRT as reference, which show the current status (they fix it on 16:27:56):
Aug 3 12:07:14 gw user.info syslog: WAN is up. IP: 221.127.134.215
Aug 3 12:14:39 gw user.info syslog: WAN is up. IP: 221.127.238.69
Aug 3 12:22:38 gw user.info syslog: WAN is up. IP: 221.127.237.15
Aug 3 12:30:18 gw user.info syslog: WAN is up. IP: 221.127.238.91
Aug 3 12:38:27 gw user.info syslog: WAN is up. IP: 221.127.238.244
Aug 3 12:59:03 gw user.info syslog: WAN is up. IP: 221.127.236.57
Aug 3 13:07:52 gw user.info syslog: WAN is up. IP: 221.127.133.13
Aug 3 16:14:32 gw user.info syslog: WAN is up. IP: 221.127.171.173
Aug 3 16:27:56 gw user.info syslog: WAN is up. IP: 221.127.239.201
Aug 4 00:00:38 gw user.info syslog: WAN is up. IP: 221.127.236.182