there should be numbers in the boxes for whatever reason yours are blank.
do as yarwell suggests and remember to untick use default snrm box.
select apply.
then select save.
optionally then reboot the router.
each notch on the slider represents roughly 0.5 db and you cant tweak much lower then 5db below the dslam target.
Andrews & Arnold
atten 50 - line dist 1.65 straight - 2.96 actual