Well that's a problem that is easy to be solved!
So your Solution is:
Press home go to the option Policy then
If you are a range class: Go to the Far attacks then
Tick "use far settings"
At the next line it sais " When target is in [ 500 ( change to let's say 10000 ) ] To melee ( Default 100)
*I'll not recomend to Attack and run option
Tick the one of the two option "Equip Weapon" Choose the weapon that you have equiped.
After that check the Attacking type on the same tab, tick the "Use spell"
a) If you are Archer just set it to use Power shot ( mp is precious ) and then he will auto attack
b) If you are Mage set your normal skill for example SH: Hurricane ,etc..
If the Attacking type don't work try to go a bit above from these setting and play with the " Skill 1/2/3/4 " options ( most of the times works if is not always )
Well cause some times the mobs will come near to you if they aint die fast your char will start hiting melee ( at least for mages ) so in order to avoid that kind of behavior , change tab and go to the Short attacks.
Tick "short attack setting"
Don't change the range leave it as it is ( default 100 )
Tick " Equip weapon " like above
And do the same set up like the Far attacks tab.
If you are melee class:
Go to the Short attacks tab
Tick "use melee setting"
Change the range " When target is in [ 100 ( change let's say to 10000 again ) ] Near.
Tick" Equip weapon " and choose your current equip weapon
Then go to the Attacking types ,Tick Use spell and choose a skill. If you are dagger: mortal blow / tank,sws: Power Strike / Bd: sting or whatevah / tyrant : Stunning fist / Spoiler/Crafter : Spoil :P etc..
If that aint working try to play again with the "skill 1/2/3/4 "options
Well that's it. I hope i helped some of you. i have boted to servers that simple auto attack from l2w don't work, but with that kind of set up it's always workin.
Have nice l2walker fun! :P