I am going to offer an alternative for you, that I think is a lot more readable/understandable.

First, note the DATEFROMPARTS() function, which you would use since you're just feeding in a year and month, that's new as of 2012. Secondly, the idea is that you only really need to find the first Monday (or whatever day we're looking for), if you want the next one then just add a week. Third, since you're using a zero-based array for @DOW, the DATEPART(dw,@1st) function will not equal zero ever, so we have to adjust @DOW when comparing.
SET DATEFIRST 7 --week starts with Sunday, default for English, but set to make sure
DECLARE
 @Year SMALLINT = 2018
, @Month TINYINT = 2 -- 1-12
, @DOW TINYINT = 0 -- Sunday = 0, Monday = 1, ect...
, @Itr TINYINT = 1 -- iteration of specified DOW
BEGIN
 DECLARE @1st DATE = DATEFROMPARTS(@Year,@Month,1)
 DECLARE @RtrnDt DATE
 --if our day of the week for the 1st is the same as we are looking for, then return it
 --we can find the next same day of the week by adding a week to that day
 IF DATEPART(dw,@1st) = (@DOW + 1) --adjusting to accomodate zero-based array
 BEGIN
  SET @RtrnDt = DATEADD(ww,@Itr-1,@1st);
 END
 --if our @DOW is later in the week, offset from the 1st by the number of days larger and add in our week iterator
 ELSE IF DATEPART(dw,@1st) < (@DOW + 1)
 BEGIN
  SET @RtrnDt = DATEADD(ww,@Itr-1,DATEADD(dd,(@DOW+1)-DATEPART(dw,@1st),@1st));
 END
 ELSE -- the @DOW is earlier in the week, subtract the value from the 1st and add in week iterator
 BEGIN
  SET @RtrnDt = DATEADD(ww,@Itr-1,DATEADD(dd,DATEPART(dw,@1st)-(@DOW),@1st));
 END
 SELECT @RtrnDt;
END

Hope that helps if you can't get your math working.