The following post is a quick tutorial how to use Trezor hardware wallet with Bitcoin Testnet.
Connect your Trezor like you normally would
Click on “Wallet settings”
Delete contents of this field and enter “https://testnet-bitcore1.trezor.io/" instead
Click “Save & Reload”
This is how the wallet page should look
Notice that in the right left corner it says “Custom backend”, this is expected. I already have 0.20 testnet BTC there.
This is how “Receive” tab looks like, there is information that you are running on testnet
This is how “Send” tab looks like, there is information that you are running on testnet
Unfortunately there’s no way for the Trezor wallet to remember this setting and you are required to repeat those steps every time you plug in your wallet into computer.