This isn’t supported, as mentioned by @lukechilds “Umbrel apps run in a secure sandbox and don’t have permission to access the devices USB ports.” BitBox - Specter Desktop - Multi Sig - #8 by lukechilds
Your Ledger should be able to connect to the standalone Specter desktop app. Make sure to select the Bitcoin App on the Ledger and then press both buttons at the same time when Specter prompts you to connect your device.