Cannot get mac to connect to Time Capsule as a drive; TC works as a network though.

I also have a MacBook running OS 10.15.7 Catalina, which is still working fine without any problem with that TC.

On this Mac.. open the Airport Utility.

Find the IPv4 address of the TC.

I strongly recommend you make it static.. so the other devices on the network don't lose the TC again.

Now go back to the iMac.

Type the ip address you found directly into the Airport Utility.. configure other.

10.0.1.1 is default in router mode.. but it can be any Private IP in bridge.

Password is the standard admin password for your TC but is usually the same as wifi password now.

If this fails tell us because it is pretty bullet proof way of getting the TC to show up.

Same thing in Finder.. use Go, Connect to Server and use IP address as

Again you will use whatever the IP you find of the actual TC.

None of this will work for long if the TC is configured in bridge behind a non-apple router.

IP can change every day.. so do go to the TC setup and simply change it from DHCP to Static. It is helpful if you can also set it by DHCP reservation on the main router but don't worry if that is not possible. Static IP should be outside DHCP range or at the very highest numbers..

Using local as domain on the main router can help. (don't worry if not possible)

When you change this from DHCP to static the current IP will stay along with most of the rest except DNS.. simply type in the same address as it used to have or a public DNS.. this is purely house keeping and irrelevant to operation.

Under internet options also set IPv6 to link local only.

This will not stop IPv6 addresses passing to the computer via the main router DHCP.

If none of this helps you need to give us full info on the network.

Main router make and model if not the TC.

How the TC is configured and best of all .. lots of screenshots of how it's setup.

Ok.. very clear I did not ask enough questions.

Please put the TC back to DHCP where you made it static.. In your case the TC is working as main router.. it is becoming so unusual that I forgot to ask.

Just give me the screenshot from airport utility including the summary.

And please also what IP the computer is getting from your System Preferences for the network in question.

Once I know what the IP of the TC is .. 10.0.1.1 being default then you can use that in Configure other in the airport utility on the Mac that won't connect.

All good then.

You should be able now to go back to your High Sierra computer.. and follow the previous set of instructions to both get the airport utility to see the TC and mount it in your Finder.. which can then allow Time Machine to work again.