Yes, reset is essential.. full factory.
Power down the Extreme.. hold in reset.. power up without releasing.. and keep holding in reset for full 10sec.. The front LED will flash rapidly when you get it right.
Make sure you set IPv6 to link local only in the computer.. this is essential.. auto is useless.
Now here is the weirdest thing of all.. airport utility must be set to ethernet.
Apple is defaulting to wifi.. and trying to setup wifi by wifi is akin to sawing off the branch you are sitting on.
Once you do that if the Airport still does not show up.. thank High Sierra.. for its abysmal dns responder.
Use the MANUAL method.. after a reset the AE will always revert to 10.0.1.1. The computer should also be at 10.0.1.2 but as long as it is any 10.0.1.x address everything is fine.
Go to Configure other. Main Menu under file.
Now type in the address.
The password is public but not required until you finish the configuration.
Go through each screen.. give the AE a name.. short, no spaces and pure alphanumerics. Password same rules but can be longer.
Wireless again short name.. no spaces.. pure alphanumerics. Use WPA2 personal security .. with suitable alphanumeric password.. and then select the correct network setup.. usually bridge.
Update the Airport and it might appear in airport utility.. dumb as it is.
