Also, are the users able to reach the server without DNS (i.e. are the IP addresse(s) involved static and communicated beforehand) and is the server itself able

Also, are the users able to reach the server without DNS (i.e. are the IP addresse(s) involved static and communicated beforehand) and is the server itself able to function without DNS? Routing is one thing which you can't do without (then you need to fallback to phone communications), but DNS is something that's quite probable to not work well in a major disaster.