A picture would really help. You are using some terms that apply to other equipment. Does your router template have a hole in the middle the size of your router plate? Or is it different? I can't quite make out exactly what you're trying to do. I have used the method illustrated below to rout out an opening for a plate. Four pieces of wood a quarter to half inch thick. Place the plate itself where you want it to go. Place the four pieces as shown and clamp the at the edges of the table. You can then use a bearing guided bit.
Mark out an opening about half an inch smaller than the plate on the wood. Cut that opening out with a jig saw before you try to cut the opening using the template.. That will allow you to cut with the mortising bit rather than trying to get a bit to plunge into the flat surface. As I think about it, I'm not sure if you've skipped that step of cutting out the smaller opening? If not, that's your problem, not the adapter.
Remove the adapter from the router, put in a mortising bit and (in 1/8th increments) rout out to a little deeper than the thickness of the plate. I added some very thin plywood between the plate and the four pieces to allow a little wiggle room to get the plate out easily. I strongly suggest you use the Kreg levelers.
You don't have to cut through the entire thickness of your table. Leaving a half inch lip for the plate to sit on will work just fine. Do use the Kreg levelers, you'll have a much easier time leveling the plate with the table top. I suggest you get a plate with the twist lock insert ring mechanism rather than the older kind that has multiple screws, which are annoying and easy to lose.
The more I do, the less I accomplish.