Hi Yen,
In order to change MTU please use MTU Property and check value using that property also.
To find out which value you should use with your peer in MTU you can try using FindMTU Method. When FindMTU completes, FindMTUDone Event is fired. In FindMTUDone Event you can find out using FindMTUDone Event MTU variable which value in MTU Property is optimal for usage with your peer.
Make sure that you are using latest 1.2.4.69 version of wodVPN. You can check version in wodVPN Version Property.
More help for MTU Property you can find here:
http://www.weonlydo.com/VPN/Help/WODVPNLib~wodVPN~MTU.html
More help for FindMTU Property you can find here:
http://www.weonlydo.com/VPN/Help/WODVPNLib~wodVPN~FindMTU.html
Let us know how it goes.
Regards,
Drazen