What should I do about not having communicated with my father's maternal uncle before his death, given my deep regret over it?

If it is difficult to maintain ties with your father due to distance or inability to communicate, there is no sin upon you, because Allah does not burden a soul beyond its capacity. You can supplicate for him, seek forgiveness for him, and give charity on his behalf if possible. Every act of worship whose reward you dedicate to him will benefit him. You must maintain ties with his children, those he loved, and his relatives, and treat them kindly as much as you are able.