This is a mix of the 2 other suggestions:

select count(*)
from Ens.MessageHeader
where TimeCreated between '2025-02-01 00:00:00' and '2025-02-28 23:59:59'
group by sessionid

All 3 options will work, always more than 1 way to skin a cat.

I would look a the SQL Show Plan to see which one is the most efficient.

Did you check the port that management portal is running on?

Given that you have just done an install, it may have changed from the default?

This should show you the port that is being used: (Thanks to Jeffrey Drumm for this code).
%SYS>d ##class(Config.Startup).Get(.Prop)

%SYS>w Prop("WebServerPort")

This is the correct answer. $SYSTEM.SQL.Functions.DATEADD() will allow you add/subtract based on any of the date parts.