The borders would be manned most likely in places where it would be beneficial for a group to have a presense. So in some places yes the borders posts are manned, but there would be large tracts of land between the manned posts where it'd be easy to slip across with ease.

Mind you, that doesn't mean there aren't patrols along the borders. Armed Technical trucks with GPMGs or even black market HMGs and perhaps just perhaps GMGs. They can't watch 100% of the border at 100% of the time, but it's possible that there might be a truck passing by. It'd be rare but it could happen.

All options for an inflitration are open. So use your imagination.

The most heavy defenses would be at manned posts. 10 foot tall walls, fences, razor wire, gun emplacements. But the best you might get in unmanned areas is maybe a 5 foot barbed wire fence, that might not even be in good repair. Gaps in the wire etc.

Have fun with it right?