In my experience, the iPad worked the best and I had credit card info saved in my wallet. That way you’re not manually entering things. The app has my credit card saved but sometimes doesn’t populate. The website never pulls up my credit card.
Also, if you don’t see your dates highlighted you can type them in on the website when looking for multiple dining reservations. Otherwise, Just click on ‘check availability’ and your dates will appear. IF they don’t (did happen) then go to my plans screen wait until it loads all your ticket info. Then go back to the dining screen and your dates will be highlighted when you click check availability. If it still doesn’t work, get on the phone with Disney Dining immediately and still keep trying.
That’s what I spent my 2 hours on, experimenting.