I'm a guest but have 25 years of professional customer service experience, as well as a good handle on managing boundaries in normal life and with customers. I'm currently in a reverse situation, btw, where the host often tries to take communications off the platform and I have to reaffirm the boundary. With high-maintenance folks like this, every time you let them cross a stated boundary, you're more likely to get a worse review from them, it's just how people are. It actually heightens their sense of entitlement and specialness, and turns you into a doormat for them to wipe their feet on as they leave the guest-host relationship.
If you're seeking such advice, I would recommend that you send them a message through the platform and say something like: "For your protection as well as my own, I require that all communications go through the Airbnb messaging platform. If I do not answer a message sent by x p.m. and after x a.m. within x amount of time, or if you have an emergency, I will welcome a text/call from you and will respond promptly. I sincerely hope you are enjoying your stay."