You are using an Airport Extreme.. Not a Time Capsule.. which is completely different.. but I guess with a USB hard disk plugged into it and shared to the network?
What OS do your MBPs use? Are they connected by wireless (ethernet??) directly to the Extreme..??
If so that is why they work
You need to HELP high Sierra with everything to do with Networks..
First off, are you running 3rd party AV or security software on the Mac? That will mess things up right proper.. !!
You need to turn them off.
You also need IPv6 set correctly.. not auto .. change it to Link-Local Only. Unless you are using a fully IPv6 integrated network. (very unlikely).
Once you sort the above.. if the Extreme still does not appear.. manually connect to it using Go /Connect to server in the Finder top menu.
Type in the address to the Extreme directly..
afp://10.0.1.1 (replace IP with actual Extreme IP which should be set statically to ensure this continues to work).
Once the Mac discovers the resource it will ask for password.. provide the password and save to your keychain.. Don't forget otherwise you will need to keep putting in your password.. well you probably have to do that anyway because of increased security in HS.
Tell us how that goes.
One other tiny bit of clarification.. The drive plugged into Airport is now and forevermore a network drive.. External drive means something plugged in by USB or FireWire or Thunderbolt. The disk is directly accessed by the computer. For anything you connect to by ethernet or wifi.. it is network drive.. and you do not directly connect.. rather you share the resource to the network.