The way I reheat a fully cooked prime rib is to preheat my oven to 350 F and then place the fully cooked prime rib into the oven on the bottom oven rack without using a baking sheet.
You can place the prime rib in a baking sheet if you want too however I find that just placing the fully cooked prime rib on the bottom rack directly works just fine.
Then I let the prime rib heat up in the oven for 4 to 5 minutes or until it's warmed up enough to where I like it.
Usually 5 minutes is more than enough to reheat the fully cooked prime rib at 350 F.
However if it's not heated up enough for you then you can heat the prime rib up for another 3 to 5 minutes but just keep a check on it so that it doesn't overcook.
You could also place the fully cooked prime rib into a toaster oven on the toaster oven rack which will also heat the fully cooked prime rib up again.