R6200v2 Poor experience (Gigabit, 2GHz & 5GHz)

I purchased the above mentioned router (R6200 v2), three days ago, as an upgrade to my home network, since my TP-Link Archer C2 stopped working and I was forced to use the ZTE H168N modem/router (10/100) from my ISP.
So I am comparing my Netgear R6200v2 to the two devices I was previously using and the comparison is not in favor of R6200v2

Setup is:
ZTE LAN1 -> R6200v2 WAN and the rest of the devices are connected via WiFi over 2.4GHz/5GHz or Gigabit LAN
Devices
iPhone 4S
iPhone 6
Nokia Lumia 930 (Win phone 10)
iPad
Laptop
Synology DS216se NAS


After connecting and setting up the new router as an AP, I run into the following issues
- SD and HD video playback from my Synology NAS freezes
- Poor WiFi coverage (both 5GHz & 2.4GHZ)
- Connected devices don't get access to internet

I Disabled the QoS and WMM options and the playback freezing seems to be resolved, but the WiFi experience got even worse. Signal is fluctuating in the same room, devices that are connected to R6200v2 over 2.4G and 5G can not access internet and I have to disconnect them and reconnect them again, in order to fix this issue two to three times a day.
The coverage of WiFi is worst than I had with the free ZTE modem/router.
I used Inssider to see the channels that are crowded in my area and discovered that I'm the ONLY ONE transmitting on 5GHz and that no one else is transmitting on 2.4GHz channel I'm using.

I contacted the support earlier today and they suggested to change the channels of both 5GHz and 2GHz
The poor coverage issue is not resolved and I'm awaiting to see if my devices can stay connected for more that 6 hours.

I feel that I just wasted my money and DOWNGRADED my home network.
If I don't get these issues resolved, I'm packing it and getting a crappy BUT RELIABLE TP-LINK...