It can be hacked immediately since it would be exposed to the world. Setting up VPN on an Asus router is extremely easy and is no harder than port forwarding. Get a RT-AC68U or RT-AC66U B1 as they aren't too expensive. If you are charged a rental fee for your modem/router I would return it in a heartbeat and purchase my own. I did the same and it paid for itself in 6-7 months.But obviously it can get hacked whenever I suppose too so,....??!!
To dumb down what you are looking at this is what I would do:
Modem (whatever is compatible with your ISP provider)->Router (RT-AC68U or RT-AC66U B1)->POE switch (gigabit & fanless with however many ports you need)->NVR (NVR5216-4KS2)->IP cams (what you've been looking at)