Post [Q] stuck in roaming

I bought my phone from Japan and I noticed recently that when I am connect to mobile data, the logo shown is "R" meaning roaming, no 3g or lte appearing. What must I do to remove it from roaming? I tried turninh roaming off but it doesn't give me any network.

If I am to root this or flash to remove docomo software, what must I do then? Please help