We aren't using our military at the border. That's against the law. The use of the military to enforce immigration or criminal laws at the border could run afoul of the Posse Comitatus Act, unless an exception applies.
It's working out quite well. They've held off, to quote you, "the most advanced best trained force in the world" for 2 years and counting. Ukraine can only do with what they have. The most modern tank in the Russian arsenal was obliterated by our Javelin missiles. Russia is not the best trained. Those soldiers are dead. They are down to rag tag groups of men that can barely drive a Russian version of Humvee. Their stockpiles of weapons are depleted.
We would slaughter them in a conventional war right now and they know it. You don't think so? You're fucking nuts. Why is this most advanced army unable to defeat Ukraine? They have no air force, barely any tanks, barely any helis. But they are keeping Putin at bay. For years. If you don't think we could whip their asses in weeks, you're nuts. You have NO IDEA our capability.