You will need to be connected to the internet in order to activate your Clover device.

After activation, your Clover device will be able to function and take transactions while offline (assuming that it’s connected to a power source). However, we recommend that you have a strong connection to the internet at all times because:

• If you take debit or credit card transactions offline, then we will authenticate the cards only after the device is re-connected to the internet.
• Your debit or credit card transactions will not be closed out until they’ve had a chance to sync to the internet.
• Any updates you make on your offline Clover device will not sync to any other Clover device or to your Web Dashboard until the device is reconnected to the internet and vice versa.
• Offline devices will not receive the latest app updates.
• Offline devices will not be able to access certain apps, like the App Market, Card Transactions app, Reporting app, etc.

After activation you don’t absolutely need to have an internet connection (if you are only running cash and check transactions), but without a connection to the internet you won’t be able to use your Web Dashboard and other powerful Clover features.

Reference Site: https://help.clover.com/faq/is-an-internet-connection-required-to-use-a-clover-device/