It sounds as if you have real problems with your Beagle. Not being able to exercise her must make weight loss difficult. Is she speyed? If so, that too can make a difference in metabolism.
I don't know whether we can help you with this but as a first port of call, if you haven't already done so, have a look at this page
which is all about helping a dog to lose weight.
Regarding the Royal Canin, I haven't been able to find the formula or the exact analysis of it but one of the websites mentioned that it was higher protein and lower fat. The theory is that it prevents the dog from losing muscle mass. Quite a few companies take this approach and it is one way of encouraging weight loss although these types of food are usually high in carbohydrates.
I wonder why you are using dried and tinned food. I can't see the point and I feel that it is likely to result in over feeding because it is not quite so easy to get the quantity right with wet food. I think you might be better off just sticking with the dry food because it can be weighed accurately. Also, wet food tends to be higher in fat although I do not know if this is the case with this particular product. If you are concerned that it is too dry and therefore unpalatable, try soaking it for half an hour before you feed it to the dog.
There is a school of thought that they should have plenty of protein and fat but smaller quantities. It is said that fat sates the appetite more than carbohydrates. I've tried this using raw complete and it wasn't wholly successful. Now I look for products with low fat (less than 12%) but whatever I choose I can never give it at the recommended daily allowance. More often than not it is something like 1% of body weight, often less. The tiny quantity has taken a lot of getting used to and I am wondering if you are having the same problem with this as I used to have?
If you would like to try another food and want some help choosing one, check out the Dog Food Directory
. Using the filters on the left hand side will help to narrow down the choice. If you need any help with this, please ask.