It's an adsl router
You will need to find the ip address of your router. Assuming you are using windows, open a dos/command prompt and issue the "ipconfig" command. Make a note of the Default Gateway. This is the ip address of your router: it will probably be 192.168.0.1
Browse to the ip address of your router in Internet Explorer (ie.
http://192.168.0.1) and you should be prompted to login. Both the default username and password will be: admin
In the configuration pages you can click on the "Broadband Connection" option and from here you should be able to check your mtu setting.