I've just dug out my swisscom SIM to see how you can check your balance online.
First there are two ways to log in:
After opening
https://sam.sso.bluewin.ch/my/data/M...ageReload=true you can either log in by username and password (given you have created such before) or by clicking on the "NATEL Login" tab and entering your swisscom phone number instead.
If you select the latter method you will receive a six-digit code valid for 15 minutes by SMS through which you can log into the customer portal. This should work for you as your Mifi can receive SMS, which you can read through its web interface at
http://192.168.1.1
Now you should see an item representing your number "Mobile 079 xxx xx xx" where it says below "Credit top up", "Set up device", "Current calls".
Just click on "Mobile 079 xxx xx xx" and you will see a "summary" showing your current balance.
By the way you can also enter, disable or change PIN codes from your Mifi's web interface.