Check your connectivity and see to it that you have stable internet connections. Note that some Clover apps have offline mode so they will appear to be functional even without an internet connection.
If you have stable internet connections and the app still says you're offline, perform a quick uninstall, then install it again. If prompted to sign in, use the login credentials you nominated in the Trezoro dashboard. Refer to this link to find out what your login credentials are.