I'm so sorry you're in this situation dingdong. I can relate entirely. I wish I could tell you the best way to deal with this - I honestly don't know. I'll tell you what I did.
My DF lived in another country when he was diagnosed. We managed to visit once relatively soon after his diagnosis, but we knew he only had a couple of months. While I was back home I would text him , usually with football scores or something I knew he would find funny. My DF wasn't particularly tech-savvy and there was no way to do video calls so it was the best I could do. Some days I didn't text him and the guilt would eat me up. I would call my DM regularly who was left dealing with everything alone, I knew she wasn't coping well all the time and there was nothing I could do to help. I felt like the worst daughter, but with hindsight I know that I couldn't have done anything differently.
Towards the end when we knew his time was short, my siblings and I descended on my DM's house for our final visit. We slept on the floor, wherever we could, it didn't matter much to anyone at that time.
I'm so grateful I saw him when I did, I would not have got the chance again to tell him all the things I did, especially to tell him how much I love him, how much I would miss him, and that I wished I had been there more. He told me that he knew I would have been there if I could, he said he knew how much I loved him, and he loved me and was proud of me. He died a few weeks later.
So my dear, dear ding; just do whatever you can, whenever you can, and don't feel any shame or guilt. You are in an impossible situation, which I am sure your DF understands like mine did. When you do see him, tell him exactly how you are feeling, don't hold back and will have no regrets.
I wish you and your family all the very best 