What you have is basic knock-off of the Knights Armament URX 3 handguard. If yours replicates the original, the way you add rail to the sides/underneath is by slotting nuts inside channels in the rail and putting corresponding bolts through the holes to meet the nuts. The rail segments were proprietary to the URX (this is an old, obsolete handguard design). There are/were airsoft replicas made by other companies, but airsoft brands almost never use the name of the real thing they clone to allow for easy search.
Fact is you are far better off not adding rails unless you categorically need tons of accessories, and there's already enough rail for anything you'd need on that handguard i.e. a small light up front for CQB (if you play in the dark/at night) and your front iron sight. These rail segments you've pictured look like very cheap M-LOK or Keymod segments which might not fit your gun at all.
