This issue is driving me insane............

I currently have a couple of raspberry pi connected to WIFI on this Archer C6

I have them set as static addresses within the router

But every few days i lose the ability to connect to the rpi webpage :-( 

cmd prompt then ping, I ping the ip address 192.168.0.10 and it shows as "Destination Host Unreachable"

Pinging 192.168.0.10 with 32 bytes of data:
Reply from 192.168.0.30: Destination host unreachable.
Reply from 192.168.0.30: Destination host unreachable.
Reply from 192.168.0.30: Destination host unreachable.
Reply from 192.168.0.30: Destination host unreachable.

Ping statistics for 192.168.0.10:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Now the interesting bit....

If i reboot router... connection is restored....................and I am able to access the rpi devices from my Windows PC again

if I look at my devices, both RPI are showing under DHCP Server as static but are not accessible from my windows pc unless i reboot

I tried setting one RPI as static and one dhcp but both are now lost/not accessible unless i reboot router!

Please can someone shed some light on this please!