I don't think it really matters what a grandparent is called, as long as it isn't insulting. I employed trained nannies for the first three years of my daughters' life – an expense I couldn't really afford, but needs must when the devil drives. I was a bit iffy about grandmothers being called nannie – maybe even slightly snobby (never let it be said). My mother-in-law chose to be called Nan and that is still how my daughter thinks of her, even though she has now died. My mother, rather bizarrely, chose to be called Grandmum and that is also how my daughter thinks of her. My eldest grandson decided I was Granna and that is how all six of my grandchildren address me. Their other grandmother is, more conventionally, Grandma.
When said daughter was born, my husband telephoned my mother and asked if he was talking to "grandma absent" in an attempt to be funny. My Ma solemnly replied, "No, she has been dead for many years", meaning MY grandmother. The joke failed.