If your BitBox01 has not been updated in a long time the latest BitBoxApp will not detect it. In that case you need to install version 3.0.0 of the old DigitalBitBoxApp and upgrade your BitBox01 firmware with that app. Once done, quit the DigitalBitBoxApp and launch the latest BitBoxApp again. The latest BitBoxApp will then detect your BitBox01.
Download version 3.0.0 of the old DigitalBitBoxApp: https://github.com/digitalbitbox/dbb-app/releases/tag/v3.0.0