Using iPad mini wifi+Cellular in Japan

Hi,

Having a trouble with my iPad mini connection in Japan.

I have an iPad mini (1st generation) cellular model bought in Australia.

I am currently in Japan and want to use the network with docomo(Japanese mob carrier )

However when I insert data sim to iPad mini, it does not recognise docomo network(no service)

I checked the sim card with my iPhone6, it works with iPhone without any problem.

Seems the iPad mini can only detect softbank network (another Japanese mob carrier) ...

How is it possible and what could be a solution????

iPad mini settings shows....

Stting>General>Abount>

Network>Not available

Carrier Docomo 24.1

Setting>Carrier (off automatic)

Softbank

Model is A1455