Family disagreements are so upsetting, but very common and to stop you having contact with your grandaughters is unfair as they are the ones who will pay the price for their mothers resentment at this time and as you live some distance from them it makes it harder, I assume you daughters partner is also somewhat controlling the decision to ban contact because of the disagreement which is unfair, he is not the biological father is he, if so he has no right to make that kind of decision, if he has a problem, then its between the adults and has nothing to do with the children, but yet is using them as pawns in a game, very immature behaviour.
Keep the contact up when the girls are at their dad's house, so they know they are important to you, it is not teaching the kids good resolution skills, your daughter should never put someone above her own children, she can only control them until they are 18, they will then make their own decisions, I'm not sure how old the girls are now.
Partners will come and go, but your children are forever and their needs should over ride anyone else, I can understand where you are coming from I have two little grandaughters and I would be so upset to, maybe there is some miscommunication between her dad and partner, but without resolving it the problem will create more of a rift, keep trying to talk but don't be door mat, people treat you how you let them, so keep your boundaries and she will hopefully come around.
In the mean time carry on with your life and I'm sure this will eventually settle, just be there for the girls and show them a loving caring grand mother.
All the best