Calling the cruise line will probably be futile. What I would do is get a letter from your doctor saying you must eat at a certain time. Have your TA submit this to the cruise line. That may or may not work.
It seems you have been assigned to the HAL open seating dining. This does allow you to choose when you want to eat. You can either do a walk-up or make a daily reservation for a specific time. If you must eat at a certain time this will allow you to do that.
Your dining time should have been set at the time of final payment. The chances of having it changed at final payment time were better than they are now.
If all else fails you can just go to the Maitre 'd when you board and explain your situation and see if he can accommodate you. The location of the maitre' d will be in the daily newsletter or guest services will know.