Are my multiple Netgear NIGHTHAWK X4S-AC2600 on network conflicting?

I had a single X4S nighthawk router connected to my BT FTTP system and was very happy with the range of the wifi and features etc.  We have a network connection that runs to an outbuilding and wanted to have a wifi system in there as well (its too far and through too many walls etc to just extend the wifi).  The wired connection has always been fine, so the distance on the cable is not a problem and well within distance requirements etc for cat 5e.  I purchased a second AC2600 (even though i dont need the WAN/ADSL side of things) as i couldnt seem to find a router/access point with the same range/features.  

I connected the second AC2600, configured it to disable the ISP side of things, set the new Static IP to be outside the DHCP range of the first AC2600, set the gateway etc as expected and everything seemed good.

The problem is it seems several times a day the wifi just disappears, we have to reboot the second AC2600 and then its fine for a few more hours.  When this happens i can access the web-based control panel but its a bit slow, if i try and access the wifi setting though it just never shows that full page.

Is there a reason a second AC2600 would not work in this manor and give these results, ive made sure the channels do not conflict even though they are out of range of each other etc?

Would there be a better netgear product for this setup?