TSQL - Finding Similar Values
you need to remember that in a like statement a _ is ANY single character, and a % is any number (including 0) characters

on your insert for StaticFilesTbl if you use the search strings you wish for in there (and bracket out the underscores) that should i believe get you what you want

INSERT INTO #StaticFilesTbl(StaticFilename)
VALUES('SG[_]%[_]123456'),
        ('TFF[_]x2[_]%[_]x3');


Also you'll nolonger need the + '%' on the end of the LIKE statement as it has everything built in already