Going offline is not unprofessional. Disappearing without notice is. The difference is one well-written message.
Say it plainly
Tell people the dates, that you will have no signal, and who can help while you are away. Do not apologise. Do not promise to check in.
I will be somewhere without signal from Thursday to Monday. Ana has everything she needs.
Leave one door open
If it truly matters, give one person the host's landline. Ask them to use it only if something is on fire. It almost never is.
