Apologies if this is the wrong place to post and I have done a search without luck. 

I have been using a Deco M5 mesh system for over a year without problem but I'm having trouble connecting a Mecool KM2 android TV box to it. The "connecting now" window times out and is unable to connect although it sees the network. The network is encrypted WPA2-PSK and password protected. If I enable Guest Network without encryption then the box connects immediately. If I apply encryption it will not connect. I have checked the password several times, tried factory reset to the box but cannot get it to connect to the encrypted network. It connects fine to an open network which is OK for testing purposes but not operationally desirable. 

I'd appreciate any advice such as; is this a known problem or is there a workaround I could try? My competence level is only just above plug n'play level and I'm stumped by this. (My ISP is Virgin btw.)

Thank you

tmac51