I will give credit where credit is due. During Obamas term illegal immigration did fall, despite the human waves that cross over the border now and then to participate in things like DACA, etc., but I think that had more to do with the bad economy if anything else. You also had aggressive enforcement of drug laws by ICE at the border which was also helpful. So to say that it fell back to Obama-era levels is not necessarily a *bad* thing.
And yes, we should build a wall. That will help "cement" Trump's legacy at trying to further decrease illegal immigration.