Unhappy [Q] Use 2g networks automatically unchecks itself in my tablet

My tab is HCL ME U2.Android 4.0.4.Now it is not sim supported.I have to connect to internet via dongle.Whenever I connect the dongle I find the option Use 2g networks only under the mobile networks setting.But can't check it.It automatically unticks.So how can I use 2g network.It drains my battery a lot.And 3g is costly in India.My tab is rooted. Anything to tweak the build.prop to get 2g networks option??Please help