As with every policy, first make sure you are looking for the right thing. You want to be protected. That is not only human, but also your employers legal obligation in any civilized country.
I'm in the same room with them, but only I am wearing a mask. Obviously this threatens hotel employees.
Now I'm not a medical professional, but this is what medical professionals have told us over the past year: A surgeons mask is used to minimize the aerosol "cloud" that contains virus droplets. In other words, it reduces the footprint of someone who is contagious. You would need to get closer to them to get the same "dosage" of virus. So we are told to wear masks and keep a distance. Both very good advice.
However, a mask does not work retroactively or magically. Your guest has already breathed, sneezed and coughed in their room for hours without a mask. If they are contagious, their whole room, all surfaces, the air itself is covered in it. Them putting on a mask now would just be a token sign of respect with little to no medical meaning. Like taking of your muddy shoes after you had a grand tour of the whole house.
The correct thing to protect you is to wear personal safety equipment that is required in this situation. That means not just any mask but a mask of FFP2 or FFP3 standard. That kind of mask is designed to protect you. Follow proper protocol, touch surfaces as little as possible, disinfect your hands, do not touch your face.
So, to sum this up: the guest cannot do anything. Them wearing a mask is only useful in shared rooms none of you lived in, like the staircase, elevator, lobby, reception, gift shop or parking garage. Their room is already contaminated (worst case) and no mask wearing from now on on their side will retroactively change that. Asking them to wear a mask or asking your boss to ask them to wear a mask is a fools errand. It does not help you, it only wastes good will on your bosses and your guests side.
So, you must come up with a plan to protect yourself. If you don't have that PPE, first thing, get it now, privately. It's probably around 5$ on Amazon or a pharmacy near you and quite frankly, you should wear it everywhere, not just at work.
Then, go to your boss, tell them why you need this kind of protection and ask how the company will provide it.
Do not talk to the guest about it. Protecting yourself is between you and your company.