I bought the same model Huawei E3131 from T-Mobile, it looks like it comes with something called HiLink, after a bit of research on how to install E3131 with HiLink on OpenWrt I found that the only parameters needed are:
Try
Interface Name: eth1
Protocol: dhcp
It looks like it actually gets installed as a network card and no settings are needed.
Also the same thing with the E3231 from Three, it comes with HiLink, no other settings needed.
This site give me the clue :)
http://bluekeane.wordpress.com/2012/09/21/how-to-use-huawei-hilink-3g-usb-dongle-on-openwrt-tp-link-wr703n/