Some guests in our hotel use routers in their rooms, and then others will be stuck. Is it because they use routers?

Not because of the router, but because they are using software that takes up a lot of traffic, such as fast broadcast of Thunder PPS. Then your internet speed will naturally slow down a lot. The solution can use speed limit software, such as P2P terminator, or you can set it manually if your general route supports speed limit.

Or rent a higher network speed.