Your mother's mother and your father's mother are both called "grandmother" most of the time. If you need to distinguish between the two people, you can say "maternal grandmother" or "paternal grandmother".

You can call them whatever is agreed on. Some common choices are grandmother, grandma, granny, nana.

I called mother's mother as "Nani" & and mother's father "Nana"

I think you you just have to accept that we say "grandmother" without saying which side of the family she is from. It's really not a problem to us. Really.