A little more information would be helpful.
1. Does this happen with other devices such as phones, tablets, or other laptops when you leave that room?
2. If so then it is probably an issue with your router itself and not the laptop.
3. If not then I would test the laptop with another router to see if you have the same result. If you still have the same result then you likely need Dell to fix the laptop under warranty. Probably a bad wifi receiver.