Telnet to your router and type the command:

> portmaptime ?

Usage:
PortmapTimeSetting   [-<command> <parameter> | … ]
-t <sec> : set TCP protocol session timeout.
-u <sec> : set UDP protocol session timeout.
-i <sec> : set IGMP protocol session timeout.
-w <sec> : set TCP WWW protocol session timeout.
-s <sec> : set TCP SYN protocol session timeout.
-f       : flush all portmaps (useful for diagnostics)
-l <List> :    List your all setting

 

> portmaptime -l

—— Current setting ——

TCP Timeout   : 60 sec.
UDP Timeout   : 180 sec.
IGMP Timeout  : 10 sec.
TCP WWW Timeout: 60 sec.
TCP SYN Timeout: 60 sec.

Just double or triple the default UDP timeout value.

> portmaptime -u 360 and press ENTER

—— Current setting ——

TCP Timeout   : 60 sec.
UDP Timeout   : 360 sec.
IGMP Timeout  : 10 sec.
TCP WWW Timeout: 60 sec.
TCP SYN Timeout: 60 sec.