Try this, I have been loking much pages on google of that topic and nathing like that...
http://koneksen.com/modem/zte-zxdsl-831/
In specially when it say this:
"...How to Configure ZTE ZXDSL 831 Modem as a Router
Visit http://192.168.1.1 with the username and password. If you don’t know the default username and password:
Username: ADSL
Password: expert03
When logged in, click on the WAN tab
Click on ATM VC and make sure, interface aal5-0 VPI = 0 & VCI = 35, MUX = LLC, Max proto = 2
Click on PPP and then a new interface
Set the new interface as PPP interface = 0 ATM VC = aal5-0, protocol = PPoE, use DNS = enable, security = PAP. Don’t forget to set your TMNet Streamyx username@streamyx and also your password. Click Submit
After that, click on Admin and then Commit and then Reboot Once it’s done you should check your Internet connectivity by loading your favorite website..."
If you enter there it woul be the "Main Page" of your connection with that modem, find anything there sinside like your IP Adress and try to change the numbers....
Try that man