The Plugin is disconnected with Mautic and shows notice of re-authorisation. How can I fix it ?

The connection is lost when the Mautic refresh token is Expire . You only need to change the Access token lifetime i.e 3600 or more  and Refresh token lifetime i.e 365 days.
You can login to Mautic Crm -> click on Setting Tab on right top corner -> Go to Configuration -> API Settings.