The guide you have on your web site is wrong for the 3200 and 2820 routers running 3.3.7.2 firmware:

The command wan ppp_mss is not a valid command.

What are the commands to set the MTU, MRU and MSS on the draytek ?
What is the MTU and MSS and how do I change it ?
• MRU - Maximum Receive Unit sets the maximum size acceptable to this device for any single PPP packet received. If a packet is larger than this value, the packet is fragmented (broken into smaller packets) before transmission.. The value is used at the start of the PPP negotiation.
• MTU - Maximum Transmit Unit sets the maximum size for any single PPP packet that will be transmitted by this device. If a packet is larger than this value, the packet is fragmented (broken into smaller packets) before transmission. The value is used at the start of the PPP negotiation.
• MSS - Maximum Segment Size is the largest size of data that TCP will send in a single IP packet. A TCP/IP header is 40 bytes, so the MSS should be at least 40 bytes lower than the peers' MTUs.
The Vigor sets its MSS deducting 40 from the value set by the ppp_mss command (see later).
To check the current MSS/MTU setting on your Vigor router, telnet to the router and then enter :
Vigor> wan ppp_mss ?
Now : 1442
The above example shows the current value is set to 1442. To change it :
Vigor> wan ppp_mss 1500
Now : 1500
You can then reboot your router; the setting is retained after power-down or reboot.