Open SSMS in administrator mode
To check current state of Maintenance mode in D365 FO -DEV
select Value from sqlsystemvariables a where a.PARM = 'configurationmode'
[If value is 0, it means It is disabled and vice-versa]
To enable current state of Maintenance mode in D365 FO -DEV
update sqlsystemvariables set VALUE = 1 where PARM = 'configurationmode'
Now you need to restart word wide web service.
To disable current state of Maintenance mode in D365 FO -DEV
update sqlsystemvariables set VALUE = 0 where PARM = 'configurationmode'