Remove both units from your network.
Reset them to factory.. hold in reset before power on.. keep holding in reset for about 10sec after power on. No files are lost on the TC.
Plug the Extreme or TC into the network.. whichever you want to go first.
Run through the setup in airport utility.
Use short names, no spaces and pure alphanumerics. Bonus points for names less than 8 characters.
Passwords same rules but 8-20 characters.
Test that it works.. if not try the other one.. at least one should work.
Assuming it works setup the other one to extend or however you want.. Extend never works well so much better if they are linked by ethernet.
No luck we need full info.
What model is the TC? Are both TC and Extreme tower model?
How old are they?
What model is the main modem? Is it also a router?
Did all this start after upgrade to Catalina? Or anything else changed. New ISP, new modem etc.
Screenshots of wherever you get up to will help.