The president made a mistake by not deploying active duty military personnel to the southern border in January 2017.
He should have kept them deployed there until the wall was built if not longer (or perhaps permanently deployed).
Virtually every other country in the world uses their military to protect their borders. Why not America?
(Suspect you may be asking rhetorically.)
Short answer: The US has used its military (or those it has commissioned) to protect its borders from both invaders and criminals - see for example American Revolution and Civil War privateers and the history of US Coast Guard.
After that, the answers become more complicated and depend on the will of our government and its current political agenda.
At perhaps the most effective point, border control would provide a sniper team, whether military or LE, at various locations to take out the apparent male leader of any group attempting an illegal crossing. Women and children would be fed, given medical attention if necessary and immediately returned along with a bill to the Mexican government.